Midnight Update: What Can I Do?

Okay — so the saga of my adorable, affection, sweet little 18-year-old kitty continues as she sleeps next to me gently purring right now. Went to the vet today because she didn't fare to well over the week with my cat sitter (not that I expected her to). Everyday is a new adventure — with the constant ear itching and lovely presents coming out of each end of her that we've been trying to treat for the past year (and particularly throughout the summer increasingly).

Dr. Bower at the South Bay Vet Group is great — very compassionate, full of hope, yet with her feet on the ground to give you realistic advice. She's gotten to know Midnight quite well even though we see other vets at that clinic when she is not there too.

A few months ago, when Dr. Bower looked into Midnight's ear it looked fine. We have seriously treated that ear for every type of skin, parasite, and allergy ailment with no success in the last two years. Today, she couldn't seem to see the eardrum which led her to believe that there may be a tumor blocking it. The only way to know for sure is a $300 diagnostics x-ray with sedation trip.

For Midnight's OTHER problems (which also seem to have become life-alteringly frequent over the last few months), when she was feeling her stomach, she seemed to think the intestine walls seemed thick. I don't know exactly what that means but the word feline lymphoma was thrown out. Again, knowing for sure would require another $300 diagnostics ultrasound. Otherwise, it could just be Irritable Bowel Disorder that none of the previous meds we've put her on have seem to taken care of. She did assure me though that there are infinite medicines.

So here's were we stand. Midnight could either have:
1. An ear tumor — the vet and I agreed that at 18 years old, she is just too fragile to go through surgery to remove it. So it would eventually be fatal.
2. Lymphoma — apparently this is just not curable although you could extend the life with chemo and other VERY costly things but that's not usually done for someone of Midnight's age and health.
3. Both! :(
4. Neither — which means more months (or years) of constant miserable vet trips which may lead to the same resistance that she's had to the most common medicines on the market.

Additionally, she's easily gone deaf since she never responds any longer unless you tap her... she has some balance problems because whenever she itches her ear now she practically falls over... but other than that and the huge issues above, she's still eating, peeing in her litter box, and wanting attention.

I feel too selfish to ever say that 'yes, her condition is affecting my quality of life', but I can't also say its actually affecting hers enough to warrant putting her to rest. The puking and yowling and straining and ear-itching cannot be fun for her, but she still remains so sweet and affectionate and content through it all.

I read somewhere that a woman was in a similar situation and she put her cat to sleep and then had constant guilt. But then she had another cat and waited and waited until one day, the second cat was in so much pain it was unbearable to watch. In the end, she thought it was more humane to put them to rest before they were at that point where they are truly suffering and miserable.

I don't even know if $300 or even the full $600 of diagnostics is worth it because the best case scenario is #4 above which is still not a happy ending. Again, I'm trying not to be selfish here but I don't have a lot of money rolling around since I'm in between jobs and even if I did, how much is too much to invest in a cat that doesn't have that many years left anyhow?

I need help, readers! What do I do for my sweet 12-year-old birthday present. She's been a constant friend and family member and, although I know that she's lead a great and happy long life, this is not a decision I take lightly. Please post what you would do if you were in my situation. I'm sad either way.