My kitty cat

Back in April I posted about Rascal and how she has become older and more sick.  I began to feed her wet cat food and she seems to rebound back.  Still about 5 lbs but was doing better.  Then, a couple weeks ago I was laying in bed and think I witnessed her have a seizure.  It was very scary even for my nursing background.  I know she has been in renal failure for sometime now and then she again had another seizure later in the afternoon.  So the go owner I am I took her to the vet, paid $300 and again learned she was in renal failure. The vet suggested a plan of care that could possibly "fix" her and prolong her life.
I decided no and just take her home.  Fixing renal failure in my 17 1/2 year old cat is probably not reversible. A couple days later she proceeded to have another massive seizure in the middle of the night and though she was going to die right there.  Balling my eyes out only thinking of her I struggled.  I called the vet the next morning and got a prescription for valium like the vet suggested.  The first time we gave her the valium it was sad but hilarious she couldn't even walk. Cutting back the dose to an amount that was more feasible. I haven't seen her have a seizure. There was one time where I didn't give it to her and she had another one so then I decided she does really need the medication.  The vet also said that valium would also increase her appetite. She now gets 1/4 tab of valium 2x a day and she seems to be doing better.  She is now neurotic about food and wakes me and my roommate up very early every morning and crawls in your lap when your eating and tries to steal bites.
Now I have a cat that needs to take valium daily to prevent her from having seizures.  I know the end is going to come but I cant let her go.  I understand the physiological aspects of what she is going though, but at this time she still eats, use the littler box and purrs, so i just don't feel right putting her to sleep now especially is the valium can help her for the time being. So now what do I do? I will always love her she means so much to me, I have grown up with her and all she is all I know.