How can I stop letting him anger me?

I'll try and explain the back story quickly. I was with a guy, he suffered from depression, anxiety and was pretty much an alcoholic, I helped him get a job, got a place for him to live to get him away from his abusive father (my ex is almost 29). I knew he had done drugs in the past but he practically lived a double live as was a cocaine addict. He wouldn't pay for rent and would come up with excuses when all his money was going on drugs. We split in January as he 'couldn't drag me down anymore' it was after this I found out about the cocaine and the lying. He has since become a massive druggie and subsequently has lost a lot of weight because of this. I moved into my own place 20 mins away, got a cat, tried settling into a new phase without cutting off my old friends and social life. He got back with his ex-girlfriend who is known to be conniving and two-faced, she very conceited and looks down her nose to everyone. I was kinda pleased he was with her as it just made him more disliked. For months on end I completely ignored him and he would walk out whenever he saw me anywhere. I talked to him three weeks ago and he finally apologised to me and he basically said his life is a complete mess, he sobbed and said that losing me was the worst thing that he'd ever done and that he had stopped seeing his ex. I had seen him out since and had been civil but naturally I was wary of him, he would come up, give me a hug, and try and speak to me but I would be polite but the walk away when it felt appropriate.

Anyway I thought there was a mutual understanding. I went to the pub on my birthday but he was there with that ex again and another friend. His ex or girlfriend hates me (I'm not sure why haha) but rather than saying happy birthday, he saw me and walked out quicker than I thought was possible. I don't know why I expected anything more but it made me so angry.

This has more to do with me than him as I know he's scum. But it frustrates me that his actions still have an effect on me. I know he's a bad person so why is he still able to disappoint me? Or upset me? I just want to stop caring. I cry over someone who doesn't care about me and it breaks me.

Because you still love him. You can't turn it off instantly it'll take time. Or it's just his lack of any semblance of decency and appreciation towards you and what you did for him. I'm going through the same, still love someone who is for the most part scum.

I think you are sad and angry about the guy you wish he was not the one he really is. See him for what he is, a drug addict and all the negative things that come with that. He's not a good person and surely you deserve better.