Shakey, this isn't nothing. This is a big deal. And I don't think for a minute that feeling so stirred up about it is attention-seeking.

In "normal" life, a scheduling mix-up or forgetfulness is no big deal. But for a T, it's a huge lapse. He rattled the frame and now there's a crack. And it's only natural that it would touch on any past feelings of abandonment, invisiblity, not mattering, not entitled to validation. Feeling uncomfortable or not deserving of being angry sounds like something from the past. Resigning yourself to it sounds like a defense against the hurt.

It might be a good idea to let yourself write it out in a stream-of-consciousness way and then bring that to your next session. That way you don't allow yourself to minimize your feelings. I suspect if you read it to him, the emotions will come back. And he needs to hear it.

You say your T has been helpful in the past, so I think you have a real opportunity here to heal some of those past invalidating feelings by working through this rupture and not avoiding or pushing the feelings away.