To not take over another thread, I thought I'd post here about your upcoming surgery, Marie. these are just thoughts to consider, don't follow my ideas without considering your own situation.
My wife had surgery, too, while we were in-house separated after she asked for (again) a divorce. I made the committment to be there before, and I was after. I worked extra hard to help, but not for the M, but because I was happy to see her take care of her health (finally!)
Your husband may have many reasons, including fear of your surgery, for saying no.
You do need to reply, but you do not need to beg. If he doesn't care, he doesn't. If his fear/sin/whatever is blocking his care, anger will not bring it back.
I would suggest: - arrange a daycare or something. The hospital may be able to help, but I'm thinking nanny or a trusted teenager who could stay overnight. Yes it costs $, but that is what happens when you are alone. If he's hooking up with another, you are alone for now.
- send the letter, but check for detached emotions. Tell him first what you are going to do, the cost, and then remind him of his earlier commitment. Tell him that you needed him for support physically and emotionally last IF you think you need to. He isn't there now, so short and sweet. Something for him to think about, not something for him to read about.