This is a long one, so stay with me. Also my first post, so be gentle!

Background: I met a girl (referred to as Jane) in my last few weeks of college. Jane and I talked for about a month and a half and really hit it off. However, I broke things off for personal reasons (I was the asshole here). About a month after that, Jane and I start talking as friends again, and a few weeks later decide to pick things up again. But, in the time that we were apart, she began talking to my best friend (referred to as John). John and I are both really good friends, and decided that no matter what had happened, we would still be friends on the other side. I told Jane that we would need to go slow to spare John's feelings (he is head over heels for her). It eventually comes out that John and Jane slept together, but her and I were not talking at the time so I am not mad at anybody, just upset that it happened. I didn't try to break up their friendship, because I think it is unhealthy to tell SOs who they can be friends with. 

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So fast forward a bit, Jane and I are together, but don't commit because I am still working through the fact that she slept with my best friend, plus John is mega depressed and I don't want to hurt him. Four months go by, and she is still hanging out with John and talking to him every day. We communicated pretty well, and she knew that I was still messed up by the whole situation. I decide that its best if we just continue as friends. 

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John and I agreed that if things ever went south between me and Jane, that he would stay away, because Jane and I had gotten fairly serious. But, the day after I broke things off, John drives a pretty long distance to go comfort her. They went back to college now and spent the first few days together. John messages me and tells me that he is going to try to be with Jane, only a week after we broke up. I told him it bothered me, and he said he knew it would. Meanwhile, Jane is calling me every night and telling me that she would never talk to John again if she could be my girlfriend. Yet she still hangs with John everyday. I never told John I was angry, but he tells me that he is angry at me because I don't want him and Jane to be together.

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This past weekend, Jane asked me to come see her, things went south and we ended up sleeping together. That night she went and got drunk with John, not sure if anything happened. This whole situation is childish, and I don't want to be involved in it. The reasons I am upset are because 1.) I feel like John is preying on Jane while she is in a vulnerable state (she calls me crying frequently because of what happened between me and her) 2.) Jane is using John to get over me, and has basically admitted to that 3.) I feel betrayed by both of them. 

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So, AITA for being angry that my best friend immediately went after my ex girlfriend after our breakup, and that she is using him to get over me?