Grant, you may come to a point when the chances of M repairing seem slim to none, but that doesn't mean you give up. This is as much if not more about reclaiming yourself, forgiving yourself and getting yourself from your knees to your feet, from your feet to standing tall. You need to do that whether the M repairs or not. Getting to your feet is a huge task, that will take a while, focus on one day, today, not yesterday, not tommorrow at a time, and do everything you can to make today a better day for yourself.
You don't have to listen and be nice about OP, it's disrespectful of her, change topic or remove yourself from the conversation.
Pushing you to set a date to give up and have a D is ridiculus. She want's out, not you, why do you have to give up? If she wants a D that bad she can initiate herself, you are not preventing her, TwinDads suggestions for responses for this are spot on. Don't be cowed into doing something you don't want to do. Its just an easy cop out for her she is after.