It's horrible when pets get sick. We've had to make some difficult decisions with our guinea pigs at times. One had an abscess that they could operate on, but he might not survive the operation, it would have cost a lot, and he had already outlived every guinea pig I'd ever had.
We passed on the operation and managed his symptoms until he got a respiratory infection and then we had him put down, but it was horrible because I couldn't even be with him as they put them in a gas chamber to knock them out. I felt guilty. The vet was rough with him, and he collapsed on her table. I should have stepped in.
But on other occasions, I've felt guilty for not taking them to the vet and they died at home. Bottom line is, vets can't do much for guinea pigs and when their time is up, it's probably up. But what to do, and how much to spend on vets bills in those final times is always a difficult decision and whatever we do, we feel guilty.
I hope you can find peace. So sorry.