I have two observations one about you and one about him.
Watch your words, you speak in absolutes about your husband - he can't cope with anything.
About him, he doesn't know healthy coping, communicating or relationship skills. You mention it that he avoids his parents and grand-parents because they devalue him. He equates conflict, speaking about his feelings, and criticism with pain and fear. Even if you don't say it he equates your money situation with him being inadequate (a criticism). He is his own worse critic. He hides behind his pain by pushing you away.
I think your letter needs to be very matter of fact, not emotional, not bringing up the past and some possible solutions. Make the tone and information all about the financial situation and if he gets upset let him know that you are not upset or disappointed with him but that you want to partner up with him to find a good solution.
Stick to the issue and a solution. Show him good coping techniques, relationship skills and communication skills. Show him how to replace all that fear with love.
M22,H45,W45 S21/18D12 Retain faith that you will prevail in the end, regardless of the difficulties and at the same time confront the most brutal facts of your current reality, whatever they might be.
