My cat is not getting better... I need your opinion

My 6 year old female cat was healthy (or seemed to be healthy) until 2 months ago when she stopped eating for a couple of days. So I had to take her to the vet. After the tests and examinations, the vet found blood in her urine, and she was diagnosed with bladder infection. Anyways, she was on antibiotics for a week. A week later, I took her for another urine test, but they found blood in her urine. So they did ultrasound on her to find out what was causing the blood. They found out one of her kidneys wasn’t functioning at all. And the other kidney was overworked and infected. They also found inflammation in her bladder. So the vet said she had Cystitis. And I came home with more antibiotics and also a new pill (There is no permanent cure for this illness). This was last week. And since then, there hasn’t been any improvement whatsoever.
I hate seeing her in pain like this. The antibiotics are making her sicker. She’s having diarrhea because of those pills. She’s sleeping most of the times. She’s not as active as she used to be. Yesterday she couldn’t clean herself, and I had to clean her. And sometimes she cries… I even canceled my vacation because of her.
I’m about to give up. I’m physically and mentally tired. She’s not happy. Should I just stop giving her those pills and just give her all my love and attention?

Is she on a special diet ?

Has the vet suggested IV fluids or at home subcutaneous fluids? This works well for helping to flush toxins from the kidneys especially since one is not functioning & the other is overworked as you say, also cleanses the bladder.

Was her urine cultured & sensitivity checked to check for a possible secondary cause of crystals or urinary stones?

What is she eating now? If she is on dryfood, a switch to a high quality canned food will help to rehydrate with the extra moisture canned food contains, also the higher ash content in dryfood tends to aggrevate the issue. You should also be adding about a teaspoon to the canned food to give more water.

To encourage more water consumption you can try a pet water fountain the constantly running water usually does work to get them to drink more.

I'd call the vet, tell him/her that she is still poorly and ask if there's an alternative medication, since the one she is on is only adding to her discomfort. Ditto what Growler said about canned food. Buy the best quality you can afford, not the cheap grocery store brands. Keep her very warm - a heating pad, if you have one, set on low and wrapped in a towel, is often a comfort. Her kidney problems are not entirely curable, but the bladder infection certainly is.
Don't worry about the fact that she is less active, this is normal when they're sick. But she must eat and get enough fluids.
Please keep us posted.