Explain you have been doing some thinking on the best way to grow our relationship going forward. Confess you both made a mistake by not setting up Ground Rules. She gets to add 1 selfish rule, and you get 1 self rule, all the rest are general guidelines. Her rule might be simple, No Cheating on her. Your rule is don't mess with my weekly pool night or monthly poker with the guys, because you have learned from past relationships, these are key items necessary for you to be there for her all the rest of the time. A relationship without rules is like pool without rules, it makes no sense. Definitely, under no circumstances can either party violate the one single rule you each set. If she can't come to terms with one rule, let her know then both parties are open to seek other partners who will be more compatible and therefore it is an open relationship, she doesn't want that. They insist on Exclusivity, so there is a price to pay for that.
I told my lady when we first met that I have only one rule, and if she followed that we could be together. That rule was simple, and it said, if it ever crosses her mind to tell me not to go play pool or to stop playing pool in any way, she had to agree not to say it, and just grab her suitcase and leave. I promised not to stop her leaving, and would only stop playing pool one time, just to help her pack and give her what money I could to go. Guess what, my lady has never said once don't go play pool, and I play about 20 to 40 hrs a week, 3-5 nights a week. I tell her a friend wants to play, she says go. No problems. All I had to do is agree not to cheat, which I was going to do any way. See a compromise without compromising or much negotiating away. If my current relationship ended, I would do the same on the next.