I need reassurance that euthanasia is the right decision

Lint was diagnosed with lymphoma in April 2021. He's 15 now. Since then, he came out of remission twice, developed diabetes and CHF (both stable/in remission), and had triaditis. In the last few weeks, his lymphoma has progressed. He was eating 5 cans of food a day and still not gaining. We switched his chemo and he had severe side effects and he hasn't bounced back. It's been a week since the side effects started and his appetite is still minimal to non-existent, despite appetite stimulants (and we have tried more than one). He's on every supportive medication. I hospitalized him for 2.5 days on IV fluids. His vet and an internist both said that there is nothing more I can do.

I'm struggling. He's still in there. He's still trying. But I know that euthanasia will save him from getting worse. I don't think I can fix this. He's down from 10lbs to 7lbs since April. He's tired, and weak. But he still is kneading. He'll eat some chicken. He seeks me out.

His sister just passed 4 weeks ago to the day, from a different type of cancer. I waited too long because she kept giving me glimmers of hope, and she passed away on her own in a crisis after being hospitalized overnight at the ER - she died in the parking lot, quickly, as I pulled out. I don't want him to crash too - but it feels like making this decision while he's still in there is giving up on him.