<human>: Can you condense the text into a few key points?

About a month ago, I broke up with my girlfriend (or, if you prefer, she broke up with me). I've been sad for a while, of course, and then I got better, but right now I'm beginning to have problems. It's a bit complicated :
Last Monday I slept at her home (needed a place to stay for the night), and she had two friends over, coming from far away. One of them just can't stand me - I have no idea why - and she went to see a movie with us this one night, so he wouldn't be in the same room as mine too long. I was already pretty depressed at this point, because I couldn't help but think it wasn't innocent of her to take him out to see a movie. Then when they came back, they found the bedroom could only host three people. So, as she "didn't want them to think she preferred to see a guy who lives in her city (me) rather than spend time with friends who came from far away" ... they all slept in the same room, and I slept alone in a bed, hearing them laugh all night. At this moment I was REALLY depressed.
Then I went somewhere else, and I progressively felt better ... And two days ago one of her friends went back home, leaving her and the other guy (the one who hates me) together alone.
Since then I've felt worse and worse. I know I shouldn't care, and she has every right to do what she wants with this guy, but the mere thought that they may be [you know what I mean, I couldn't find a way to express it] even as we speak makes me gloom and wanting to cry. I guess the problem is with me (even if the fact she mostly doesn't talk to me when she has guest doesn't help me) and I've got to accept this ... But how ? Have you got any advice on how to get better, or to make peace with these facts ? Even advice on what to do when I'm depressed would help ..
<bot>: My ex-girlfriend is probably sleeping with a guy who hates me, and I have a hard time accepting it. What to do ?