Sounds like he was using you during times where he made major eff-ups and other people didn't want to be in contact with him. I am so very sorry that you have gone through that.
I truly believe that there are some people that are addicts and the addiction makes them selfish and do bad things… Then I believe that there are people who are selfish by nature who also happen to be addicts.
I was in a similar relationship when I was in my late teens-early 20s. The person did not have any addiction issues, but they were very similar to this guy you were talking about. Trust me, you're better off now. If it makes you feel any better I'm now in a relationship with a wonderful person.
Once you cut off contact with him, in a few months, you're gonna look back on this and think how grateful you are that this person is not in your life.
Don't think that all people with addiction issues are like this. Drugs and alcohol can make us jerks, but when we work on getting better we also work on ourselves. Hang in there and keep posting if you need support.