I don't think your T will be pissed at you if this is the reason why you would postpone the appointment.
But this isn't the focus, right?

I'd say, keep your appointment, protect yourself. I understand that it's really hard, I always have a horrible time when I have to do this with my mother. But you always put others first, and it seems like the problem wouldn't only be to disappoint T. You need that session for your own well being plus you offered a compromise to your mom. It will be hard, but you are you and you can do what is better for yourself, whether others acknowledge it or not. You would have to do something that is incredibly hurtful to you but people won't be sorry if you miss your appointment. Only you can take care of yourself.. easier said than done, I know.
Thinking of you.