I can relate too. I have a child with a disability, and don't work because I need to care for him. My marriage is a good one, thank goodness, but I have wondered (when I'm po'd at my husband

) what on earth I would do, if our marriage did go bad. I guess I would tough it out like you are doing. Is marriage counseling an option? Or perhaps going to counseling yourself?
I had a hard time accepting my son's condition, and last fall I started going to a counselor, and it was a huge help to me. I went in the mornings, when my kids were in school, about every 2 weeks, for a few months. My husband's work had a program that paid for it. Once I got myself back, I was able to be closer to my husband, and be a better wife and mother.
One more thing, I have hired a couple of college girls and trained them to do therapy with my son. Maybe you could do something similar, but just for respite? The first few times they come, you would be there to show them how to handle your children, and let them both get used to each other. Then leave them for short periods, and then longer periods. Just a thought.
I feel for you, and I wish I had some really great advice.