Oh Laurie, I'm so sorry about your Kitty Girl. I lost a foster kitten a few years ago to FIP & it took me a very long time to deal with his loss. I found him & his 3 siblings in a cardboard box outside of a McDonald's one day while I was heading in to lunch. Lucky for them (& for me) I worked at a vet clinic at the time, so I rushed them all back to work. They checked out as physically healthy, so I took them on as foster kittens until I could get them spayed/neutered & adopted out. They were all friendly, happy & playful, with my Little Man (who eventually died of FIP) being my favorite. When they reached 2 pounds I brought them in to be spayed/neutered. The other 3 had their surgeries that day, but when they did the pre-op on Little Man they found he had a low-grade fever & put off his surgery. Nothing else seemed amiss - he was still playful, loving & cuddley. His fever persisted for over a week, and by the end of that week he started slowing down. He prefered sleeping to playing and the spark in his eyes that I loved so much was gone. The vet finally tapped his abdomen & found that it was filling with fluid. They told me that it was FIP & that there was nothing that I could do. It broke my heart (and still does, to this day), but I had to euthanize my Little Man at 4 months old! Like you, I knew that it was the best thing for him & I was able to be with him for the procedure, but it was at that point one of the hardest things I have ever done.
Losing my Little Man & caring for the other animals abandoned at the vet clinic inspired me to go on to a career in animal rescue. I am now an adoption counselor at an animal shelter & I get to help change the lives of abandoned & unwanted animals on a daily basis. I do it for myself, but also in memory of my Little Man and the other beloved fosters I was unable to save along the way. I have learned that I cannot save every animal, but that I can provide care & comfort to all those that I come across. Sometimes, making a very sick animal feel valued & loved in their last days on this earth is the only (& best) gift I can give them. You may not have had a long time to spend with your Kitty Girl, but please know that taking an abandoned kitten & giving her love, a home & a feeling of of worthiness is an amazing gift and one that your girl surely felt lucky to have. Without you her life would have been entirely different and may have ended much earlier than at 11 months if someone had not found her at 1 week old.
You will always miss her & wish that the two of you had more time together, but maybe knowing that you did everything that you could for her & gave her everything she ever needed will help you in the long run. I am so sorry for your loss & wish there were more I could do.