I can relate. My dear old friend Mo-dog was healthy all his life, until one day we found 2 golf ball sized lumps in his neck. Turned out to be lymphoma. We took him to a specialist and he went through Chemo for 9 weeks, he took it really well too.
After the 9 weeks, he was in remission for 9 months, then it came back, and we did another 4 weeks of chemo. This time it was gone for good.
A year or so later, he started drinking non-stop, turned out he had diabetes. We gave him 2 shots of insulin a day, and a strict diet. Although he went blind from cataracts, he still enjoyed a quality of life. We still traveled with him and he would make one circuit around a hotel room and memorized it's layout.
Then one day my wife calls me to come home, as he had a seizure. I came how and held his head as he had another, with tears rolling down my cheeks for him. We took him on that final ride, and even though it was the right thing to do, after he got that last shot, he tried to get up, which broke my heart, and made me second guess our decision.
****, the water works are on now, friggin pets!
Good luck, you will know when it's time, even if you second guess yourself like I did, you'll know deep down it was a GREAT and amazing last gift that we can give our beloved family members, our pets.