As what everyone else said, with your family background I would deffinatly be careful. My dad was an alcoholic, as was his dad, and his dad's brother (Not sure what that makes him to me) had problems with coke. My uncle on my mother's side served in the air force, and while stationed in thailand (I think it was at least), got into heroin and it really wrecked him, although he is clean now and just drinks occasionally. While I don't have a problem with alcohol, I do with opiates and benzos. I have friends who use xanax recreationally without any signs of addiction. What I guess I'm getting at is that addiction is that if your parents and other family members have had drug problems, you are more prone to them. But you probably already know that.
If you were to ask the majority of addicts, they all had lines that they crossed after justifying it to themselves (Such as what could happen with the line you said you have drawn in your mind with heroin and what you did with amphetamines). I don't think anyone wakes up one morning and decides theyre going to go get addicted to heroin, it's a gradual process.
Alot of what addiction is, at least for me, is psychological. Which is why weed can deffinatly be addictive. If you feel yourself smoking it for something other than having fun, and using it more compulsively or because you feel you need it, I would watch it. Use caution with it. Enjoy yourself, but try and recognize when it's no longer enjoyment, and more of just doing it because you always do, if you understand what i'm saying.
Synthetic weed is terrible. A friend of mine (Not SWIM shit, this was actually a friend, not me) had a horrible addiction to spice. She used for maybe 6 months, and after quitting had withdrawl symptoms very simmilar to what I experienced with roxys. For months afterwards, she experienced intense depersonalization, thought she was going insane, awful depression, etc. Before using it, she was a really normal, happy person. Since that, she hasnt really been the same (Although, I only knew her for about a year before this happened, but from what I knew of her then, it's what I saw). She uses anything to get high nowadays, binging on dxm, concerta, etc. and is just pretty down most of the time. The same thing has happened to a number of people I know who used spice alot. I personally only smoked it once and hated it. I'm very against it, and would strongly advise you not to smoke it anymore and just stick with actual weed. I'd hate to see that happen to anyone else.
I guess what I'm trying to say is that you know yourself better than anyone, if you recognize that your developing a problem, to cut back or quit, or seek help.