I'm sure there must be some kind of warning (she looks all down in the mouth, etc) before these talks. Just go do something. Let her start, say, "I understand that you feel that way. I'm sorry. And I'm sorry, but I've got a few things I need to do" and then get gone. I really don't care if it's somewhat rude. Unless, of course, you want to take a more direct course and say, "know what? I really have too much on my plate to deal with more of this right now. I understand you feel this way, and really, I agree with you that it's best for you to go your own way and sort things out. Can we please just not talk about this for awhile."
You cannot be lonely if you like the person you're alone with. Dr. Wayne Dyer