Which is better, a chance to say goodbye or the blessing of never knowing death was coming, never having to be afraid?. I've had two people in my family pass the same way. One was an Aunt who laid down for a bit of a nap and was gone. The other (also an Aunt) went out to shovel her walk, fell over and was gone. Neither could be brought back, and at 91 it's likely that recovery from a massive heart attack
would leave her with very little quality of life, if any.
Obviously you loved this person, and it wasn't your fault that she passed on. Unlike the movies, once a person completely flat-lines (no electrical activity) they can't be brought back. You could have had all the training in the world and not been able to save her.
If I had to choose to a way to go, it would be like that. With someone who loved me having a last visit, and never seeing death coming. Please don't blame yourself.