Hello there. My name isXXXXX and I am very sorry to hear about your sweet Bella. I know this is a very difficult time and for you and there will always be "what ifs" and "i should haves" in these situations. But you have to find peace in knowing you rushed her to the hospital and got her good care immediately. Also you gave her a long and wonderful life with an obviously loving family. In Bella's case it sounds like no matter how much money you spent or what you put her through you most likely would have been making this hard decision. I have recently had a very similar case with a dog around her age. To make a long story short my patient suddenly had seizures and lost the ability to walk. He was taken to a specialist where he also declined and eventually needed assistance breathing. An MRI was done and he was found to have a brain tumor that had grown on his brain stem. They believe it was there for a while but finally had reached a size that it became detrimental. That is the most likely thing that happened to Bella. From your description and her age she most likely had a tumor in her brain/brain stem that reached a point that it was pressing on vital portions and causing this rapid decline. In the case of my patient surgery was not an option and all the specialist agreed that the best thing was to put him down. So in Bella's case surgery was most likely not possible and no other treatment was possible that would be considered appropriate because none of them would most likely return her to a quality life. So your decision was very loving and humane and was the best for her. Although extremely difficult. I hope this helps a little bit and gives you some peace. The other causes such as infections in the brain (encephalitis) (meningitis) usually occur over a period of time and not so rapidly. One of the only things I have seen that causes this to happen so quickly is a tumor. So please do not feel guilty or beat yourself up. You did the best thing for her and again the most loving and caring thing for her.
I hope this helps and eases your mind some. Please let me know if you have any additional questions or concerns. I would be happy to help. Please do not forget to rate my response so I receive credit for my time. Thank you