Addiction is a complex mental disease. It really is a disease, which some people are susceptible to and some aren't. In myself, I look back and realize I've always had an addictive personality; as a kid, I was addicted to video games, then other games, then an online text-based multiplayer role-playing game that I got insanely into, it was my other life, I played 8 hours a day if possible.. Then it turned to drugs, or rather, I turned to drugs, got really into them, and found myself addicted. However my drug addiction(s) have proven to be different from my other ones, in that drugs alter your brain chemistry and to use them habitually causes long-lasting changes away from homeostasis which alters your life and your entire perception very dramatically, which makes drug addiction much worse. I'll share my story since you asked, the parts relevant to this question anyway.
I've struggled with mental addiction to drugs/mind altering in general, and also a full-blown long-lasting physical addiction to opiates that was nearly life-destroying. When I was 17, I started smoking weed and drinking. The alcohol was never a problem for me; I mean, I was reckless with it, but I just did it when I was partying, I never felt I needed to do it or felt compelled to do it every day. But the weed, I did that every day from the beginning. At first it was innocent... I had great experiences and when I was doing something else for a day I wouldn't smoke. Soon though I found myself thinking about it a lot, and I started smoking constantly, trying to stay high all day every day. I'd bring it on family vacations where I was hiding it and no one else was doing it with me. I'd do it to go to school, I'd do it to go to bed, I'd do it to wake up. Then in college I started using psychedelics, infrequently. I tried more and more drugs, ephedrine, speed, cocaine, minor opiates, I even tried heroin once. I did MDMA a good dozen times. I still didn't think I had any sort of problem, but in reality, I was altering myself as often as I could, because I felt like life was more fun with drugs. I had become addicted to the experience of altering myself in some way (this is known as poly-drug addiction). It didn't matter what it was. I smoked weed all the time but it was mainly because it was the most available, and all my friends did it just as much as me and I lived with 4 of the biggest potheads I know, with 4 more upstairs. If I didn't have weed I was fine if I had something else, but if I had nothing I thought relentlessly about how to get something. There was no physical withdrawal at all, just a behavioral modification of drug seeking and being obsessed with drugs in my mind.
After college I started doing psychedelics VERY often, up to 4 days out of every week or more. Most of the time it was low doses for a minor alteration to my day, but I also tripped hard plenty of times. So by this point I had the following arsenal of types of drugs I was picking and choosing from daily to alter myself, including: weed, alcohol, psychedelics, dissociatives, and stimulants of all kinds, and also opiates (but I will describe that below because it was my physical addiction and my worst problem by far). I still struggle with this sometimes, many years later, though I have gotten it pretty much under control (usually). I still use drugs including weed, alcohol and psychedelics but far from every day, and fortunately I am done with opiates. It was hard for me to recognize I was addicted because of self-rationalizations, and the fact that it wasn't really having a negative impact (or enough of one for me to notice) on my life. When I stopped behaving this way it didn't take long to feel fine without them, mostly it was the craving to alter myself that was difficult for me.
Now opiates, on the other hand, became a physical addiction, and that was a whole different and more destructive thing. I started using kratom (an opioid plant that is quite powerful if you have no opiate tolerance or prior periods of abuse) when I was 20/21. I absolutely loved it, it was just another drug to get high on but one of my favorite. I started using it every other day, then pretty quickly every day. At the time it was virtually unknown in most of the world and I had no idea it was addictive, the place selling it was touting it as a non-addictive replacement for opiates, and I loved the feeling much more than other opiates I had tried. Soon, kratom became my favorite drug by far, it outshone weed even, and I was a dedicated pothead then. I did it every day or even twice a day for months, and loved every minute of it. This was my honeymoon period. It felt so good and benign, I had absolutely no clue that I had become physically addicted because I was on it daily, I never took a break from it. No part of my mind felt alarm at what I was doing, I felt I had found a miracle plant.
Then one day my next shipment didn't arrive. I was disappointed, but I just thought it would come the next day, so I smoked weed and waited til then. I woke up the next morning feeling the most ungodly anxiety and a feeling like I was going to jump out of my skin. It seemed to crawl with a restless energy, and I felt weak and helpless. It scared me so bad, I didn't know what it was but I felt so horrible and I couldn't motivate myself to do anything. I was in college then and I skipped my classes that day and just curled up in the fetal position on my bed. At some point, I realized I was thinking obsessively about getting that shipment later that day (I skipped classes so I wouldn't miss it as well as because I felt so out of sorts), and then I realized: I was physically addicted to this plant. I still didn't think of myself as an "addict" though, even though anyone looking in at my situation would have realized it. It took me years to get to the point of acceptance of my addiction, addiction messes with your mind so much. There is an incredible amount of rationalizing to yourself. It's the addict part of your brain trying to maintain the addiction by convincing you it's okay. That's the most difficult part of addiction, is that you can't trust your own thoughts and feelings. People usually don't realize they're addicted until they're a ways into it, which is why it's so dangerous.
Anyway, the shipment never came that day. Frantic at this point, I checked the tracking number and it had been sent to the opposite end of the country. Over the next few days, it kept getting sent all over the place, it made no sense, especially since the place it was shipping from was literally 50 miles away from where I lived, it usually arrived the next day. I was so frustrated and in such terrible discomfort. Every day the mail truck would come by I'd run up to them with my stomach fluttering and with wild eyes I'd ask if I had a package. Finally, on the 5th day, it came. That morning I had woken up and felt 100% better and happy finally, but as soon as it came I wanted to feel that wonderful feeling again so I tore open the package and made myself a fat dose. I still didn't think I was an addict. "I'll use it more responsibly this time, I'm not addicted anymore so I can do this" I said to myself. That didn't happen, I immediately used it daily or twice daily again, just as I had been, but even slightly more often. I had a number of more cycles like this, withdrawal when I didn't get my next shipment, and increased use when I got it again, and after about a year and a half I admitted to myself I had a problem. But, I still rationalized it to myself, saying "I'm young and I'm enjoying myself, I'll quit before it becomes a REAL problem". I kept on like this for 6 or 7 years. Gradually it got worse, it affected my mood more. I became crushingly depressed over time because I felt like a slave and was increasingly disappointed in myself all the time. Around year 5 I realized it had become a "REAL" problem and I started trying to reduce. I made many attempts to quit, all unsuccessful. I was scared at this point, scared of my own brain, but I still thought of myself as a successful person, just with a monkey on my back.
At about year 7, I decided I would use poppy seed tea (basically oral opium, containing morphine, codeine, etc) a few times to get past kratom withdrawals, and quit for good. My wife at the time helped me through this, and also stopped doing kratom herself (she never became addicted to it and was fine without it and would take breaks regularly with no issue - until then we used together so there were no secrets). But I liked poppy tea even better, and thus began the saddest chapter of my addiction. I began doing poppy seeds every other day, then every day. I loved the highs and had a honeymoon period with that too. But before long I was much worse off than before, addicted to a much stronger opiate, and a full opiate. My life spiralled down and down. I began to lie to my wife all the time because she no longer supported my use. I stole from grocery stores to get poppy seeds. I almost got caught up doing that a few times but just moved to a different source each time (I am a likable guy so even when I did get caught they just made me pay for it and banned me from the store rather than calling the police). I rationalized this all to myself and didn't see it like I would see it now, as a seriously risky and uncharacteristic act. It was all to support the opiate habit. Over the next 3 years I ground my relationship into the dust, destroyed my self-respect, and eventually, in late 2013, literally wanted to die because things had gotten so bad. I went $40k into debt over these years because my opiates of choice were available legitimately so I could use credit cards, and in fact could not afford it without credit. It wasn't just the circumstances, it was also that my reward systems and neurochemistry were so blasted from 10 years of opiates. It felt impossible to be happy, even when I was on opiates I was sad and angry at myself and felt self-loathing, but at least it was masked then and I felt physically good. When I didn't have them, the withdrawals were soul-crushing, absolutely horrible. I just couldn't quit, I temporarily did so many times, then I'd relapse, hide it from my wife, she'd start to trust me again, then she'd find out and it would drive another nail in... she'd help me quit again with a bit more resentment, repeat process ad nauseum.
In mid 2013 she left me, but we still lived together for about 8 months (which was very painful, confusing and horrible). In early 2014 she moved back with her mom, and I realized that the relationship was actually causing me a lot of pain and had been all along and I was masking that with opiates. But I was still badly addicted. Finally this April I took a flood dose of ibogaine, having tried everything else, and like a miracle it worked. It seemed to dig the opiate addiction right out, I can honestly say I haven't had a single craving since and, of course, have never used them since. That was 6 months ago. If I hadn't gone through with the ibogaine, I don't know where I'd be, maybe I could have done it on my own but the ibogaine provided a huge help to me for that. So that's my story of opiate addiction, I hope it has been revealing to you in some of the thought processes that go into physical addictions. Addiction is mostly mental, but when something is physically addictive it actually changes your brain so the mental part becomes so much worse. Even the physical symptoms are because your brain is unable to reach homeostasis without the drug, so a variety of physical responses manifest as those parts of your brain scream out for the it.
Holy crap that was long...