GM, I don't see what you told her as a specific ultimatum, but rather how you felt about her exploring other relationships. It's up to her to decide what she wants, but you are going on w/your life and living it each and every day.
Mlcers do rewrite history and the history can go way back, some even bring up things that happened when they were in their late teens/early 20's.
Keep your expectations at zero and set your boundaries. You have more control than you think...but you'll discover this as you walk the path.
Hang in there!
Sit quietly, the answers will reveal themselves when you least expect them to. The past is gone, the present is a gift and you need to focus on today, allow the future to reveal itself when it is ready.