Am I lost? Do you really need to set boundaries with someone you're dating. Isn't it an unspoken rule of trust that you shouldn't have to say, oh and btw please don't cheat on me. She cheated on you and told you. Ballsy move on her part, and I'm glad you remained calm. You can remain calm and not take it on the chin though.
And what's this bullshit about analyzing her psychology. What does that even mean? What, she's emotionally unstable now that she's had a foursome? Grow a pair and move on, UNLESS you want to be in an open relationship.