Have you considered that your reaction to her in this situation is enacting a repetition compulsion with your abuser, and that is what MAKES it impossible for you to see it as a joke? And in the same way that granite's T was not very polite in returning her letter, your T is similarly choosing the client's best interest, ultimately a positive therapeutic outcome, over the "simple" route, which would be to give you the apology you want.

You know how "they" are always saying, "the only way around is through". Maybe the only way to your apology from her is yeah enlightenment.

You can't forgive her; you can't forgive your abuser. You say what your abuser did was not such a big deal. And now you say what T did was a joke - that sounds like not such a big deal, on the face of it, UNLIKE your perp's actions, frankly. So what do you object to, in both cases? Where does/did the humiliation come from, and what will truly fix it, once and for all?
Are these the kinds of things you and T discuss?