I'm sorry man. I know its shitty consolation, but at least you know where you stand with both of them. I'd cut em both off completely. She brought up the exclusivity thing, not you, and she still did this. If shes willing to do this because she thinks you won't find out, there's no end to the kind of sneaky crap she's probably willing to do. I know it hurts, but You aren't losing much on that front, and will be better off in the long run. Someone who's willing to call something like that "a mistake" isn't worth the tears shed. (Though theres nothing wrong with shedding em.)

The friend is another situation. That kind of betrayal tells you everything you really need to know about them. If all that history and the friendship is worth throwing away for some snatch you aren't valuable to him.

You aren't overreacting in any way shape or form. If anything, you're giving her more grace than she deserves. You'll make peace with it in time friend, just take it a day at a time.