I'm so sorry for your loss and the tragic turn your baby experienced after so many months of 24/7 care. Please know you are not responsible for what happened. There were accidents (and I use that word lightly because what you describe is negligence on the part of medical professionals) however you did everything right, for months and years - the entire life of your "bunny". The love, determination to fight and treat and give the best chance to your baby was above what most do. Unfortunately all our strength, will and love cannot control the future. I truly wish it did, because we would both still have our babies.
The reason I say this is because while you are in the throws of intense, raw emotional loss right now, and questioning everything with a sense of guilt + anger + sadness, all of what you're feeling is normal and part of grieving. It is a way we try to make sense of and control the past. Your baby was suffering two very serious ailments at once. When we are desperately trying to treat and save our fur-kids, our focus is so set on that mission that we are not prepared in any way for when a sudden turn in their health occurs, and that causes a lot of shock to add to the grief. I'm glad your baby was purring and being held in your arms during those final moments, know that all your little one felt was secure and loved and that is the best gift you could give. I'm so sorry you didn't get more time together. Please be kind to yourself and allow your heart to mourn your loss. My best to you.