KJ, Do you see your H on a fast track to D? Is he in a hurry to make a decision?
Your letter is asking him to think about his needs and desire, and to discuss them in therapy. This seems like a reasonable request to me.
The letter also has some elements of an ultimatum in it. If he continues to see the OW, what are saying you're going to do?
My advice would be to frame the letter in terms of what you are trying to influence right now. What small steps are you wanting your H to make at this time? Otherwise, it becomes an ultimatum letter, telling him to cease and desist with the OW or else you will do X. What approach do you want to take? What kind of letter do you want it to be, ultimatum or request?
CL
CL 53 W 54 M 20 yrs. 03'-09' Separation + Old Patterns + GAL 10-14' Piecing
"The Master allows things to happen. She shapes events as they come."