Hi Glen,
What Bill says is what works best for me. I have written similar letters for t and tended to think too much about them, now I just keep writing until Im done. It can be edited later if necessary. If you feel its getting overwhelming then you could just do as much as you can. Maybe writing it close to the time you have therapy would be an idea so that you can process the feelings it brings up for you in your therapy.
It has been a good way to get to feelings and other perspectives for me, but if I fear being overwhelmed or bringing up too much at once then I just do as much as I feel able to at the time. Its always an option to do it again, I have written some letters a few times.
Good luck with it.
Rustam