Wow, Marpeth, you have so much on your plate and (seemingly) so little joy in your life! I really feel for you. Finding some personal happiness should be your top priority right now, IMO. Whether it's through resuming studies/career, doing volunteer work, taking up a new hobby/passion, meeting new people, going on group holidays (perhaps those organized by your Alma Mater), writing a book about the "caregiver syndrome," or any number of other things -- you need to reclaim some joy in life.
As for your marriage, it sounds pretty dead to me. Any chance you could slowly, methodically work toward a separation? You might both be happier for it.
What is more devistating is for children to live in a house where the parents should be divorced. Kids aren't stupid. My best friends parent waited until they went to college to get divorced and all three children asked why they waited so long. They siad they stayed together for the children. Each one of them said , "you stayed together for us so you could be miseralbe and fight around us?! Thanks." He said it was hell. His parents should have been seperated of years and their life living together was terrible. So don't use your kids as an excuse. If you thik you are doing them a favor your not.
I feel very sorry for you. It sounds like you have reaaly drawn the short straw. But what is worse is staying another day in a bad situation.