Losing My Kitty Buddy

My beloved 15 + year old Kitty was recently diagnosed with CRF. Even though it's only been a short time since she was diagnosed, she's already been through a lot. Right now, she's suffering from a really bad kidney and urinary tract infection and so she's not eating and she's been throwing up a lot. She's got the best care I can afford (I've already spent nearly a thousand dollars on her treatment in the last couple of months alone), but I don't want to prolong her suffering just because I can't bear to lose her. She's my furry little kid. I'm devastated at the thought of having to put her to sleep but can't bear to see her like this. She's not her wonderful, happy, talkative, playful self. She's withdrawn, lethargic and seems to be depressed.

Everybody says "you'll know when it's time" but how?? She goes through times when she seems almost normal again but then she gets even more sick the next round. I'm so not ready to let her go but should I keep holding out hope that she'll have a few good days in between the bad ones? I just don't know what to do anymore.