Sad. All of this is very sad. Having said that, I firmly believe my own kids are NOT responsible for my happiness or success in life. That is my own job. Fortunately for me, so far I think I've done it pretty well. But even if I hadn't, I know deep inside my own gut that my kids are SUPPOSED to move on and build their own lives. That's what you have done. That is what you are SUPPOSED to do. Do not feel guilty about it.
Keep calling. Keep caring. But accept that your mom is responsible for her own life and how it turns out. It really is up to her to make it work out. It's sad that her first marriage didn't work and her current partner is dying. But these things are a part of living. When life kicks us in the teeth, it really is up to us to pick ourselves up and find a way to go forward. We don't get to kick back and lay all our troubles on our kids and expect them to fix it for us. Do let her know that you care, but don't consider for a moment going back and "taking care of her". Instead urge her to make friends, explore new interests, and generally take charge of her life again. Heck, I know folks in their 90s who still do just that: They volunteer or they are invested in their churches or they start a bridge club or a book club--they keep on keeping on. My own mom was 84 when she passed and she was in the midst of writing a murder mystery and taking an online class to help her learn how to do it. She was living in a nursing home at the time because of a heart attack that left her wheelchair-bound but she never quit.
People are stronger than they ever realize and they will never realize how strong they are IF everybody rushes in to take care of them. Sure you make sure she has resources and you keep active tabs on her BUT you also encourage her to build a life for herself and interact with the people around her.
JMO
Rusty
PS If it matters, I'm 64.