Try to make the distinction between how you feel and how she feels. Are you just finding the nursing a tremendous strain and the pain YOU feel hard to bear? I think this is important when deciding what to do. It's always a help to have the opinion of a close cat-loving friend who is less involved emotionally. I've always leaned on friends a great deal at such times to help me decide just how my ill cat is feeling.
If she's having off days, then a good day, let her make the most of the good days. Is she still purring? If so, she's having happy moments, let her have as many as she can before you make the final decision.
I always err on the side of waiting until it's obvious my cat's life is nothing but suffering. I have to live for years with the decisions I've made. Guilt is a natural part of grief, so I try not to make it worse.
I've recently had to have Polly pts, but I was certain her life was over. I miss her and hate the way she died, but I know she was at the end of her life.
I hope Maddy and you cope with her illness for a long time yet. Take some easy time for yourself otherwise you'll be overwhelmed by the stress of nursing her.
Lots of good vibes for you both.