How do I know when it's time to put my cat to sleep?

I have a 14 year old tortoiseshell cat who was diagnosed with kidney disease last year. We have been keeping her going with low protein prescription cat food, thyroid medication, and potassium powder. She's had 4 or 5 urinary tract infections. Her last one was a bad one. Basically we treat the infection, it goes away with up to a month of antibiotics (they do a month for the bad ones) and then we wait for the next infection. I should also mention we're in the midst of some financial problems. We're a little more than half way through a 5-year debt-repayment program. We're managing, but things are tight. Just in the past week, both of our cars required significant repairs. It was almost $1000 for both, which doesn't seem like much, but it is to me right now. Anyway, I've noticed over the last couple of days, my cat has a runny eye. It tears up a lot and it seems to bug her. I tried wiping it with a warm wet cloth and she seemed extremely uncomfortable, almost like it hurt to put any pressure on the area between the side of her nose and her eye. She's also been sleeping under the beds a lot. I think she's feeling lethargic again. She's lost some weight since this disease started and sometimes she perks up, but I am noticing she doesn't seem to be enjoying life anymore. By that I mean she used to love to sit at the patio door and watch the birds. Now she seems tired and goes under the bed. I'm getting to the point where I am considering having her put to sleep, but I feel guilty. I feel like I am giving up on her. I don't like watching her health deteriorate, but I also can't afford many more $200 vet bills. In the last year I think we've spent about $1000 on her, probably more. In your opinion, and I'd love to hear from fellow cat lovers, am I being selfish if I have her put down? I have made an appointment for this coming week, but I don't know if I should keep it. I love my cat to death.