Soooo I ended up getting back with my ex a month or two ago... month and a half really. I still love her, she's great, etc etc. But I can't seem to get over the fact that she fucked my best friend (not once or twice, consistently, we weren't dated, it's a long and complicated ordeal i don't want to get into). It still tears me up inside to think of it, and now that he's back in town it's kind of hard not to. Nothing's ever brought me to such an angry/anxious/depressed state as that whole ordeal did, and it's still imprinted upon me. I feel like this can only end badly, yet kind of don't care. Should I just leave her despite the fact I love her? I really don't know what to do.

Unless you want to go through counseling or something to work through it, I'd be surprised if you're able to work through it and you'll not be truly happy.

If you can enjoy what you've got for now, then enjoy it whilst its lasts.

Use your time "with her" now, to get over her... Then when she fucks your boy again, you'll be able to just pick up and move on.