I’m so sorry for your loss. What a beautiful bird. I’ve had birds for many years; have 2 cockatiels now. Sometimes they get sick and no matter what the vet does, it doesn’t stop the inevitable. Often the vet is just treating symptoms without knowing the real cause. I’ve even has a necropsy done before but the findings were inconclusive. So don’t feel bad about not doing that. I know how hard it is to not know. I’m sure you did everything you could for her. From here, I’ve learned that guilt often occurs with grief no matter what you do. I’ve lost birds suddenly and was devastated with many tears and much heartache. I even lost a cockatiel that was only 8 months old; the grief was unbearable at times but somehow I got through it. I hope knowing that you made her happy gives you some comfort. You loved her and she loved you. The bond you shared will last forever. May your heart heal in time.