Jeep, my friend, I'm so sorry. That pain is crushing. My own father died when I was a teenager and I never had the chance to say goodbye.
I know how much you are kicking yourself over the situation with your ex. Please stop doing that. I can tell you as a mother, she understands and doesn't fault you AT ALL. More than likely she wishes she had been able to do more for YOU. She would have done anything to spare you that pain. She understands so you need to let that go for you and for her. She's obviously not well. She needs to be able to focus on her and not worry about the guilt and hurt she'll see in your eyes every time you look at her. Give her that peace.
You were a young man with your grandfather. All young people think, it's never going to happen to me or there's always tomorrow. As an adult, you know this isn't true. You'll make better decisions this time.
Spend the time you have with your mom. Create good memories that will last a lifetime. Don't feel guilty or regret what happened in the past. You said yourself you can't fix it. So why focus on it. The only thing that will happen is you'll diminish the time you have left now. Make the best of it.
M:41 H:43 T:26yrs M:19 yrs S:15 D1:14 D2:9 Living together but separated