What is with my ex’s behaviour?

Hello Everybody,
Its the very first time I write my story online, but this one is worth the post.
Briefly, I was dating a guy suffering from depression, for 7 months. I was a huge change in his life and he loved me so much, but unfortunetly he never gave as much as I did: So I kind of felt alone. In summer, when I travelled, he was extremely distant and mean, and I got involved into a sexual affair with a guy I didn’t even like. And I regret it. I was honest with him. Told him the truth (without blaming him). And he cried. A lot. But stayed with me for about a month. After a while he decided to shout at me on the phone: Insult, curse me, and threaten me, then break up. We met again at a concert, which turned into disaster: He kissed me while drunk then pushed me etc.. And after this I couldn’t go to any event. I heard that he was taking drugs, getting really drunk, didn’t want any serious affair with any girl, and still cursed me, talked behind my back, revealed my secrets to people: Revenge basically.
He explicitly told everyone that he didn’t give a damn about me anymore, but i always felt like he still did. He was often at places we could meet but Friend would tell me it was to meet other people and other told me it is because he wanted to stumble upon me without efforts. Recently, I decided to go to an event-Where he went ( we have Friends in common). At the event, he didn’t even say Hi, I would catch him looking at me, and he even got closer to the chair I was sitting on.
And this is 6 months after the breakup, and I recently heard he got drunk at a party, my school organized. He changed a lot and is starting to go to music concerts he doesn’t even listen to for exemple. It looks like he would do ANYTHING to feel better. I need advices, on what is happening in his head.
Please. Thank you.
What is with my ex’s behaviour?
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