I don't know why your T ends the session every time you have a flashback. Do the flashbacks always come at the end of the session or does your T end the session in the middle because you had a flashback? I would get really upset if my T ended the session in the middle and if this is what is happening you should definitely ask your T about it (if you feel comfortable). I can definitely relate to things in therapy putting you in a bad mood though...sometimes for me things dont really come up until the end of the session and then I have to leave with all these feelings.
I wish i could offer you more advice ... but this is something I have alot of trouble with myself...take care XX