Sorry your hurting, I don't think you're overreacting because everyone deals with friendship/relationship losses (however long or short) in their own ways. That said my own experience tells me to tell you to ~try~ not to be too bitter and assign it to material things - I know that's difficult but it won't help and may make it harder to connect to people in the future. I will try to explain why here.

I may also get jumped on a ton for this but I would say to avoid dating younger fresh-out-of-high school kids from now on. I know it is only 3 years but the mental gap between 18 and 21 is HUGE and it's obvious she is still figuring herself out, with the long-term string of ex's, bisexual curiousity, jumping around, and emotional inability and I'm assuming kind of immaturity (not a shot at anyone, literally just looking at the story and the ages).... chances are she may not stay with your buddy long either. So as angry and unfair and hurt as this all seems, and you are valid feeling that way, also remember it is perennial.

I'm not good at relationship support. Basically, you sound like a kind person and one that someone will appreciate and also return that kindness with once you can move past this. I know it sucks in the short term and I'm sorry I can't help more with that. Also sorry if this feels preachy, not at all the route I'm trying to go with this. Basically take this experience and use it to how you may 'filter' relationships in the future? Agh I'm bad at this :,( Basically scream and cry and do what you need to get out that tension and hurt, it is not overreacting, be upset, and then try to continue on when you get to feeling a bit better. Keep throwing yourself at social situations, it sounds like you've come a long way since your time in high school and I know that shit isn't easy :, )