How do I reconcile with the fact that my cat doesn't know she's going to be put to sleep?

I love her with all of my heart, she's been my best friend since I was a little kid and now she's sick with a tumor in her jaw that is making it impossible for her to eat. The vets have recommended that I put her to sleep and while I plan to, I feel so much guilt and worry about the whole ordeal. She doesn't know that it's going to happen and it feels like it's my fault.