My ex is so over me and it's only been like a month

Me and my ex boyfriend, we went out for 2 years. Such a strong relationship, I can honestly say I never thought for a second we would ever split up. But we did. So we split up, I was totally and utterly in bits over it, didn't think I'd ever get over us breaking up, in reality, I don't think I quite am, but he obviously is. When I went to give him his stuff back, he patted me on the head???? WHAT! 2 years, and I get a pat on the head like I was a bloody dog or something, seriously. Anyways, I left, in floods of tears. (typical) Went home. And about a week later, it turns out he has had a spit roast?!?! Honestly, I wanted to kill him, but there was nothing I could do. He got his mate to tell me, that is low. Did I mention I am going on holiday with him in a couple weeks, which doesn't make situations better at all. (We planned this when we were going out) So yeah. Couple weeks after that, I find out he's going out with someone new already??? Is that fucking normal? Did I mean anything to him? I am so gutted, I'm so angry at him but I need to be friends with him because of this holiday. If I wasn't going on holiday with him, we wouldn't even talk anyways. He is now taking all kinds of drugs and drinks nearly everyday.. Ahhh, I know I shouldn't care, but I do. A lot actually.