I think the tendency is to look to ourselves for the fault - that way we can have a definite reason for why things happen...but sometimes the fault really isn't with us, or at least not solely with us.
I wouldn;t like to offer any specifics based on a few posts but clesrly your ex has issues. I don;t think it's healthy for you to internalise those or take responsibility for them.
I think sometimes too we want to blame the drug not the man...but whatever the cause the relationship obviously made you unhappy.
As for why you fall for these guys - I'm no psychoanalyst but a good counsellor could be valuable in helping you work that out?