A few months ago, before her regular trip back home to the east coast suggested opening the relationship while she was there for a week. We had talked about open relationships as a concept early in the relationship and how we both thought they were not our thing at all. A year later, before her trip to mexico she actually asked if she could open the relationship while she was there, at the time I agreed (having a sort of mind left body moment. not good, i know) thinking there was no way she could actually pull it off, and when she came back it turns out she hadn't done it. Now a while later she suggests this on her trip back to the east coast. I felt immediately pretty bad about it and it took some convincing with a caviat from me that there could only be kissing and absolutely no sex, hoping again that it falls through. But no, it turned out she had arranged a hook up with a friend of a friend and and after a few days of gut wrenching conversations after she returned from the trip she finally admitted that they did everything except penetration and that she couldnt guarantee she wouldn't want to open the relationship again in the future.
I love her, and as far as I know she loves me, but the past few weeks I have been feeling extremely low. I tried to put off my feelings and just get back to where we left off but I can't shake the feeling that I'm not enough, and when i see her laughing and hanging out with guys in our friend group it sends me spiralling, often ending up in conversation about it and I just feel awful being so jealous. She's suggested I go to therapy, which I have followed through on and will start in a few days. Maybe the therapy will help, but the feeling so much pain and jealousy so often is really killing me, and the thought if breaking up with her enters my mind almost daily.
Sorry my writing is a bit scattered.
TL;DR Gf openned relationship during a trip, wasnt totally honest with me afterwards and now my brain is broken