First of all, it's not 'swinging.' According to you (since we haven't had your partner's point of view,) she's not being honest with you, and she's hooking up without your knowledge and approval. That is wrong, and in almost all cases, it leads to the kind of hard feelings you are having. I'm very sorry for you for that.
Now, where do you go from here?
1) You say you want out. You've make it clear in your message that you feel she's cheating. Okay, so what's to stop you from taking her out to a coffee shop and telling her you're out? It's going to be hard - I assume you are very fond of this woman, if not in love - but assuming you'll eventually break up with her no matter what, it'll be hard then, too. Doing it this way just brings on the sadness earlier and saves you a lot of pain.
2) If you feel that there's any chance that the relationship could be saved, go to with her to therapy under the following condition: Until you are finished with the therapy, neither of you will have relations with anyone else. If she doesn't accept that condition, it means she's not invested enough to have therapy work. If she breaks the condition during the therapy, it means she has a problem that couples therapy isn't going to fix. But . . . if both of you keep your word, and you're able to work it out through therapy, then perhaps you'll have a loving companion for the rest of your life.
As I said before, this really isn't a swinger's question, it's a relationship question. I wish you the best of luck.