Jenny, I am so sorry to hear about Buffy. Your situation is bringing tears to my eyes, it is so similar to what I just went through three weeks ago with Libby, my 17 year old cat. I've had her since I was in college and she was with me though everything.
Same as Buffy, Libby got sick abot a month and a half ago and stopped eating. She'd eat people food for a while then nothing at all. She got so weak and it was just killing me to see her grow weaker and weaker. She wanted to be in my lap all the time, always near me. The vets' diagnosis was cancer. He prepared me for the fact that she was not going to pull out of this one.
I cryed everytime I had to think about taking her in to the vet to put her down, it was such a truamatic decision and I put it off on a daily basis, tormented between my own guilt at needing her to be with me and yet wanting to know if she was in misery or pain and if I should be the one to make that decision. Back and forth, back and forth. I'd run home from work at lunch time just to hold her for an extra 1/2 hour a day because I knew the end was near.
Well, I finally made the decision to put her down. I made the appointment for the following afternoon because I wanted just one more night to cuddle her. Well, Libby ended up dying in her sleep in the morning and it was a sad sad time. I was so thankful to have that cat all these years, I did not want her to be miserable anymore, so for her I was glad it was over.
Jenny sometimes making that decision is the right thing to do. I am so sorry that you are going through this, I know it is tormenting fromfirst hand experience and its still very fresh in my mind.
Well, I can tell you this: Libby will meet Buffy at the Pearly Gate and show her around, k? Hang onto the memories, they will always be with you.