I felt the most embarrassed and ashamed ever in my therapist's office when I told him I pull out my hair compulsively. I have trichotillomania. I had so much shame about it that I didn't tell him until we'd been meeting for 2.5 years. I was so nervous that my shirt was literally stuck to the couch as I stood up after session.
(I'm not sure if that's the kind of embarrassing you meant.)
We've talked about it several times since then and have thereby reduced the intensity of the feeling to where I can say about two sentences every few sessions and only sweat a bit lol

What was the hardest thing you've told your therapist? Or one of them, if the hardest one is too hard to write here.