Could you write it in a letter to your therapist? That would prevent you from being on the spot and give you time to breathe if you need a break in talking about it. Perhaps give it to him a day or two before your appointment, and maybe even move the appointment up a bit, so you don't have to wait the three weeks and have this tear you up inside.

I also suffer from lots of depersonalization and derealization and talking about it in therapy has helped. I believe it is a necessary step to move forward, and I have learned strategies to help myself stay focused. Sometimes therapy is painful. It's not pleasant, but it's part of the process of healing, especially from past trauma.