How does one forgive oneself after having been through mom's painful death?

Back story... I had been living with, and taking care of my elderly mom for 6 years in AZ, she didn't have dementia or anything, but needed looking after. A couple years ago she sold her house, moved in with my sisters back east, so that she would have more people looking after her....(which I was good with, made sense to me...)
Last November she came for a visit, had Thanksgiving together and all, except for a slight stomach issue she did real good, feeling good.... was set to fly home the next day, (this was on a Saturday) then, later that day I noticed something was really wrong. She didn't look right, weak, and pain in her left arm. I called 911, they took her to the local hospital, I followed and when I got there, the ER nurse said she's actively having a heart attack and is being flown to the city hospital that has a cardio unit. She made it there, got a stint put in, survived all that surprisingly, her being 88 years old and an ex smoker. My sister came in from back east the next day, (thank goodness) we knew mom would be in no condition to fly anywhere, so was planning on spending Christmas together here in AZ. Mom was still weak, not eating much, but was doing better and was being moved to a less critical room right by cardio unit (this was on Tuesday now). One nurse came in and talked to all of us about the next steps, a couple more days in the hospital, then to PT till she was strong enough to travel. Mom was disheartened, saying she didn't want to spend another night there, that it frightened her, she just wanted to go home. We tried to be encourage, said she had to do little steps. We did get her moved, and she started saying she was in pain. The nurse tried getting her BP, but couldn't. A team came in, they rushed her to radiology, found a blood clot moved from her groin, (where they went in with the stint) to her leg. They were getting her ready for surgery, we were just talking to the Doctor who was going to do it, when there was a code blue in ER that she had to go to (she looked like a deer in headlights, knowing she had to leave our mom). Now, mom is in serious pain, we don't know why no one gave her anything for the pain, took everything we had for me and my sister to hold her down to keep her from taking off her leads, kept telling her they'll be back soon to take her pain away, and she just writhed in pain saying it hurt, it hurt. Finally they came back, put her under, and took her up to surgery. We knew the risk, we knew there was a chance she might not survive, and when we heard the code blue in surgery, we knew what happened. They tried, but mom didn't make it.
Now I'm living with blaming myself, (cuz she died on my watch) the "what ifs" (I should have spent the nights with her, and encouraged her more maybe she would have made it) and seeing her in pain over and over again in my mind.