Abdominal mass in a cat.

My husband and I put down our Ivy today. It was rather sudden and unexpected and now I am troubled and regretting our decision.
Ivy was 11 years old, a playful troublemaker that had no interest in listening or obeying you. But none the less we loved her. This past tuesday I noticed she was acting "off". She wasn't in her usual places, and I had not noticed her eating or using the litter. Although things were difficult to tell because we have another cat. Wednesday came, and still nothing. I even physically took her to her food and nothing. She was not much of a lap cat, and we didn't really get to check her out very often. I noticed that she had lost quite a bit of weight and that her back end was very bony. She was matted, and had some bald patches, and she smelt of urine all over. She wasn't wet at all and there was no evidence of her going to bathroom.
Her drinking had been fine, or so I think. But she would only drink out of the running faucet, toilet bowl, shower.
We took her to the vet today with the understanding that it was probably a kidney issue and were prepared to make an appropriate decision. The vet told us the usual, need blood work, urine test and she also mentioned hat there was a mass in her abdomen. She said it could be the obstruction in the bladder or a tumor in the kidney for which we would need an X-ray too.
We are not in the best financial situation, and I didn't want to waste hundreds and hundreds of dollars to just tell me that any treatment plan would cost the same and may not help. After an hour in the office we came to extremely difficult decision to euthanize her. Our reasoning for this included,..
Pain
Weight loss from 9 lbs in 2008 to 6lbs now
Finances
Longevity with or without treatment
Behavior, although the change was very recent
Stress, we have another cat at home that is usually the docile one except when Ivy came home from being somewhere ( vets, grooming ) then she just tormented her for days
Ivy also had her own anxiety, didn't like leaving her own environment. She was deemed "handle with caution in her file from an early age". She would hiss, cry, get quite vocal and would usually soil herself when we went to the vet. She needed sedation to be groomed and needed to be sedated before being euthanized.

So here the time has come, the vet assures me ( with hesitation ) that we are doing the best thing. It's done, and then I asked to feel the mass she found. She felt for it again and told me she was almost sure that it was a very large kidney tumor. That calmed me for a bit.

Now, I am emotionally distraught over our decision. What if it was just a benign mass, what if it was a thyroid issue or diabetes that we could have managed quite easily. I guess my regret is not having any testing done to tell us something as least. I feel guilty that I pushed for this because of money issues. She was our cat and I never though that I loved her as much as I do.
I'm sorry for the long winded story. My question is given that there was really no proof of anything, did I make a very serious mistake?
Thank you in advance, Katherine

Thank you for your response. I'm also wondering whether or not you think that it may have been a thyroid or diabetes issue. And what the treatments plans for those would be in best and worst case .
I'm still quite a mess about our decision, mostly because I have no closure.
Awaiting your response, thanks.
Katherine