I have been in this type of situation before. Looking back, I realize that those people were just in a different stage in life than I was. I wanted more and had worked to improve myself and my life, whereas they were still stuck in their drinking, always looking back. I moved up and on with my life. Sometimes I still see or talk to one of those guys and do you know, they are STILL drinking and drugging, complaining about the same things they always complained about: everyone and everything but them.
It really was senseless for me to have stayed with any of them longer than the few months it took to realize what tbey were REALLY like. But in each case, it was too late, I was already emotionally invested in them and the relationship. Now I know better.
The trick for me, really, has been in always looking forward. Focusing on my future and what I want for myself and my life both short and long term.
There is no person in this world that will hold me back and hold me down like that again.