Susan Mary - you are not 'bad' at this...you are torn and confused.
Let me give you my own personal experience.
4 years ago, my rottweiler was diagnosed with cancer. I had surgery after surgery to try and stop it from spreading. It went into remission for a short period and then it reappeared...this time there was nothing I could do and had to accept the fact that the end was near. I went to my vet and asked how much time without further intervention. He gave me 1 month with meds. He told me to enjoy this time with him but regardless of his level of energy and 'masked' health..he will deteriorate.
Keep in mind that with the surgeries - it did buy him 2 years.
1 month later, Ben died. I did whatever I could and in the end, he was being called to cross the bridge. I knew that morning.
All this to say, you will do whatever you feel is right.
Ask the vet if there is anything that can keep him comfortable if infact this operation will only buy him a few months. Keep in mind about the recovery time and if it's really worth it.
I wish I had the magic word or answer for you. I am so sorry.