I can really relate to this... I have wanted to cancel so many appts before due to feeling "too fat". But your T is there to help you. He/She is there to help you, not to judge you, and I'm sure you will feel much better once you've gone and settled in to the appt. Just be open and honest with your T.

As for how to handle the shame... for me, I try to remind myself that what I am seeing (or even feeling) is what my disordered mind wants to see/feel. It does mean that is is the truth. Perhaps your T could help work with you on your self-esteem? I know I really struggle with this too, and it's something that me and mine work on a lot.

Best of luck to you!