what should I do about my bestfriend and my ex?

I'm not trying to sound rude upfront, but you should communicate. If you don't communicate, then your relationship takes a big dip. And you don't want that. If your girlfriend really loved you or cared for you, then you would KNOW she would have understood and you would have trusted her to tell her. Why you didn't I don't know. You should talk to your ex and tell her about your situation. Even if you two are broken up, you should explain to her and tell your side of the story. And if you are uncomfortable about seeing your best friend with her then you should talk to him about it. In my opinion, friends shouldn't date other friend's ex's because problems arise and there's messes. And drama. So if he was a real friend then he wouldn't do that to you. But, like I said. Communication. It's important and may the force be with you! Good luck and I hope I helped.

This guy is not your best friend. Best friends don't do that. The lesson you should learn from this is that you've got to talk about your depression. Failing to do so destroys any chance you might have of making a relationship work. Your so called best friend and your former girlfriend may well have gotten together while trying to cope with your issues. You should talk to one or both of them.