It's a tough call to make. I talked to my T about it very slowly. Just a little at a time so I could judge his reaction. I started with saying I need to say something. That let him know what I was about to say was hard and he should give me some space to find the words.

It took months for me to get it all out! Some people can do it in a session or a letter. Some take years. Trust yourself and your T (in small doses at first, but not all Ts deserve your trust).