I'm sorry to hear you're having such a tough time. To me it sounds like you are not a loser; you're a person with problems that you're trying to deal with, and that is the opposite of loser-y IMO. Could you maybe schedule an extra appointment with your T and talk about how you feel after therapy? Sometimes I have definitely felt a "vulnerability hangover" as you call it but maybe there's a way your T can structure things so you don't leave feeling so awful.

Also, I'm curious what kind of mindfulness stuff you were trying? For me exercise helps a lot but meditation by itself doesn't seem to work.