My girlfriend is staying over at a guy's, that she had feelings for

That's quite a lengthy title, but I guess it's fitting for this lengthy story. So let me begin, early June my girlfriend Sam met this one guy named Mike. Mike is a streamer through Twitch that has a very similar demeanor to another friend of ours. She got excited and joined his Discord server to try and do a meet up and see the two world clash. Between this time, she started noticing that he was a bit sad, so she went to him to try and help him feel better. In this they start talking about very personal things, he talks to her about his divorce and how he might lose custody of his kids. In this whole discussion she finds out that many of his emotions are very similar to hers, and that they even share BPD. Fast forward to late August, in the middle of a call she breaks down. She tells me that her therapist told we to keep this from me, but she recently fell in love with Mike. She was very confused because she really did not want to end our relationship, but she really like this guy too. So what her therapist told her to do was to make us both fly out to LA, California (he lives in northern California, I live in Puerto Rico) and to go see her. From there the therapist stated that she should pick whoever she felt the most comfortable with. I was really hurt and sacred of losing her. I wasn't really hurt about her liking someone else because I understand that, you don't really get to chose who you end up liking like that, it just sort of happens. What hurt me was...