Please don't think you are giving up on her. You are not. Sadly, you are accepting reality and doing what is best for her. I understand the feeling completely, I truly felt that I had killed my best friend when I had to let my first girl go when she was diagnosed. With my last one, the vet said something that I found helpful, which was that even with the highest dose of pain meds, my girl would still be living with daily breakthrough throbbing pain. It didn't make it better, but saving her from further pain did help a bit.
I am so sorry that you are having to go through this heartache.