Sounds to me like she is already having an A. Please do not beg plead or even try to discuss your relationship with her right now,unless she brings it up first. If she brings R then Validate her feelings and do not argue with her and make it about you as that just makes it worse.
Have you read the book Divorce remeady? If not please get it and read.
You need to "act as if". That means act as if everything is fine with you. Go out with friends, do things for you do not tell her where you are going, be mysterious. Also do not set up times to see her. Let her come to you and then only do some of them with her not all. These are some things mentioned in the book.
Has she mentioned getting married before? has this been an issue with the two of you?
Maybe she is using this guy to get you to marry her also. Ya never know.
JAK
You don't get to choose how you're going to die. Or when, you can only decide how you're going to live now. ~Joan Baez