Hello, I would like to say sorry for your loss, and animals such as dogs have so much sentimental value.
My dog had to be put down due to also having cancer - it was just suffering way too much.
I slowly gave the dog a nice stroke, said my goodbyes and his body slowly began to rest - very saddening moment. The dog was bought on the day I was born and so I had grown up with him.
I was extremely unhappy for days but then I just began to have these happy memories of when I used to take him to the park and we'd play fetch and I remembered how he was no longer like that nearing the end of his life.
I did miss him - but I knew I had done the right thing and should remember him in a good way, it all comes in time, don't worry.
Rest in peace to your dog my friend, I know how it feels...