She went away for a week. She caught feelings. She had a panic attack and told me and says she feels terrible, that she didn’t cheat. That maybe she’s just always been more inclined towards polyamory.

First, usually you don't catch feelings for someone else just like that. It often comes hand in hand with someone having had doubts for some time, but not communicating properly.

But I’m not, Reddit. I’m completely broken. I don’t know what to do

Take a step back from this relationship and figure out what YOU want. If you want to stay, what kind of behaviour/actions do you want to see from her?

Keep in mind, even if it's complex, you're never in too deep. Yes, a break-up will hurt, but you need to think about the longterm.