tdotb, I think now you have done everything you possibly can to try to get her to talk - the letter, trying to talk with her about it, etc. - and she still hasn't responded in any way.
She's either deeply depressed or has fallen out of love with you. If she's depressed, she is the one who needs to seek help. I'm not a great advocate of brinkmanship, but it seems to have got so bad that perhaps now the only way you can do anything is if you shock her into seeking help by threatening to leave (if only for a trial period).
The other possibility you have to consider is that she has fallen out of love with you. This I know is where you don't want to go. But you're only 40 and you have possibly another 40 years ahead of you. Do you really want to feel like this for all that time?
Most of us know what it's like to be in a relationship where we love the other person more than they love us, and after a certain point it becomes completely destructive - to everyone. The lack of physical affection is soul-destroying. (I've just spent a week apart from my dh because of his work, and I know my stress levels and blood pressure were higher all week just because of the lack of physical comfort.)
I know there is a child involved, but there are plenty of happy children with separated parents.
I also know that when you're in a relationshihp you think you'll never survive without the other person. But then, by some miracle, you do. (And there are hundreds of women looking for a lovely 40-yr-old!!)
I don't know whether you'll want to do any of these things, but if you do nothing else, at least go to another doctor - not your completely useless-sounding gp - and get a recommendation or a referral for counselling. Or go to Relate. Just talking with another sympathetic human being will help you sort a few things out in your mind, and perhaps help you make some sort of decision about the future, however minor.
sorry, that's v. long, but heartfelt.