"I saw you flirting with that girl. I am so sad and upset, I wish you would treat me like that. Do you have anything to say for yourself?" Again, her comments tend to paint me in a corner and take a position which will be lose/lose.
It is very difficult to respond to accusations in the heat of the moment - I have found this to be helpful to self-regulate my emotions:
1. Pause.
2. Pay attention to my emotions (what are you feeling?, don't try to change it, just be aware)
3. Pay attention to my physical sensation (is your blood pressure up?, do you feel knots in your stomach?)
4. Pay attention to your body posture and language (crossed arms or legs, eye contact, hunched)
5. Half-smile. (not a smirk or a full smile - because that could be seen as invalidating - the kind of smile you give yourself when you are feeling sad and want to show care, but not that you are glad about a situation)
6. Validate and cheerlead yourself. (in your head - tell yourself you are doing the best you can do right now)
Then respond. Here is what might be a good response. "I can understand that you would be upset if you thought I was treating another girl better than I was treating you. That would feel horrible. I like treating you in ways that would make you feel good. What things did you see me doing that you would like me to do for you?"
(this is easier when not in the heat of the moment)