You need to simply define your priorities and apply them to your life.
What is more important to you? (you don't have to answer anywhere but in your own head)
Using drugs, or being physically fit/healty?
Go over the 'benefits' that drugs provide you, along with the consequences (which are usually more ample) and decide if the difference is more positive or negative in your life.
You have to first establish and believe that drugs are 100% undeniably making your life worse, before you can really move on to ridding them from your lifestyle. And the good news is the expecation of being physically fit off drugs in the future can help motivate you a lot now in getting off them.
I too use to be skinny when I was younger (like 17) and got even skinnier when I started abusing speed. If I showed you my first drivers license I look like Kevin Bacon in that Machinist movie... basically anorexic.
After I got off speed I had 5 years to build my body, learn about diet, and put a lot of work into my appearance. I had a 6 pack, wasn't too big nor too small, just a nice average size on an ultra lean figure. I was basically ripped to shreds a few years ago.
When I picked up my opiate habit I was able to maintain my workout intensity and overall diet for about the first 6 months. But the longer I was on drugs the more and more stressful going to the gym and eating right became for me.
Another thing I like about my physique was the crazy amount of pussy I use to get lol. I'm a short guy, and women don't tend to go for short guys, but as far as I was able to take my body, women didn't seem to care anymore lol. That was when I learned its not really height they're attracted to, but the false sense of security and protection from taller guys. A shorter guy whos in excellent shape can provide that same false sense of security.
Not to mention it made me confident enough to also approach them in the first place.
Now, I have put on weight, and although I'm not "fat" by societies means, I consider myself a fat as fuck just in comparison to how I use to look. So its hard to shake the feeling of always feeling fat now.
If I was abusing speed instead of opiates right now, I'd be rail thin just like you. But opiates tend to make me softer looking, which is whats going on now.
I think about it everyday basically, how badly I just wanna be healthy and back in the gym. But I've largely shut that priority off untill I'm able to get off drugs, and shut off that priority first.
You CAN get back in shape. It DOES take effort, but the best part is you've already learned a lot through trial and error.
It took me about 2 years to look how I did before, but the older/wiser I get I now realize I could get that same look in only 9 months or so. Cause I don't have to make all the mistakes I made in my diet the first time around.
So don't think you're that far behind. I DO suggest though that you stop the drugs if being in shape is that high a priority for you.
And don't fantasize too much about it, sometimes when you dream too much about 1 thing you subconciously tell your mind you can't have it, which makes it harder to actually get.
Like when I tend to go off in my mind and daydream about banging Meghan Fox. Its exciting to think about, but I'm really telling my mind "the reason I need to dream about it is cause its never gonna happen". Which is another reason its usually not good to fantasize about being clean either. It creates an odd sort of prophecy where it always remains just a dream.
G/luck bro you just gotta be patient and a little more deliberate with your lifestyle. If you want it bad enough I'm sure you'll get it again, same as I will. =]