I do understand your feelings of guilt I really do. But not only did you give Mitzie a home when she was 13, a decision that few would make knowing an ageing cat comes with health issues, but at the end you eased her suffering. Our dog left 10 weeks ago. I've churned guilt continuously. The circumstances are similar. He had pancreatitis a few years ago. It reared it's ugly head again in his last week. One day he was rolling in the sun, the next sick and in pain. We tried for a week, he also had epilepsy, a heart murmur and at 15 was slowing down. After a week and lots of tests we had to make the heartbreaking decision. To begin with I felt it was best. Then I started questioning should we have tried for a few more days. The reality is though that pancreatitis is so nasty, and was probably indicative of pancreatic cancer. A confirmation of that diagnosis would have meant he would have to endure horrible tests and for what. Just to know he might have had a few more weeks. Those weeks would have diminished his spirit, he wouldn't have been able to eat, the strain on his heart too much.
For Mitzie it seems very much the same. You got her there as soon as you could. You questioned everything. The heart results really are telling. I can't be sure but it feels to me if you had not made the decision and driven her back the trauma could have caused all sorts of problems. You ended her suffering in the kindest way and that takes courage knowing how it was going to feel. At least you weren't selfish and it is so easy to be like that when we are terrified of the loss.
You did the right thing.
I'm sorry you have to deal with this.
Love you forever and ever and I will be there for you xx