Re: How to help others to understand? Desperate.
I'm so sorry that you are going through this. Honestly, I don't know if anyone can understand, other than the people who are going through this exact same thing (such as us here on the board).
Writing the letter to him is something I would have done with my BF; I think the counseling idea is great. Having a 3rd (neutral) party to explain things to him might really help.
He is probably very confused by your ailment (heck, we are confused by it, and we are the ones who suffer from it). I would imagine that someone who has never suffered our condition cannot come close to ever understanding it.
I really wish you the best of luck. Just reassure him that you love him, and that it has NOTHING to do with him or your feelings. I hope he is able to understand. It's so hard to have the added stress of a spouse/partner not understanding. Believe me, I deal with that too.
I hope you feel better & things work out okay.