I'm glad the meeting went well and that he'll be there to support you and the children. Now, turn your focus on to you and your health issues and let the rest of the stuff settle down. You've got to get yourself in a good frame of mind for the testing, etc. and keep positive. Stress can play a cruel game w/your body and you need to get as much rest as you can and build up your emotional and physical strength for what is to come in the near future.

If you write the letter, put it in a box for safekeeping. I, personally, would not give him the letter. You had a conversation yesterday about your relationship and you've advised him that you want him to be happy. That should suffice for now.

Keep the focus on YOU!

Sit quietly, the answers will reveal themselves when you least expect them to.
The past is gone, the present is a gift and you need to focus on today, allow the future to reveal itself when it is ready.