I'm sorry to hear this! Also sorry for everone in this thread
I had to make this decision twice with our dogs, the first time I was 17years old, the last time was last year.
My last dog was a little pug/pekingese mix, 18 years old, and he couldn't walk/drink/eat alone anymore, was blind, became deaf, his heart condition became worse (he had this tho since we got him from the shelter with 7years) etc.
It was all very hard.
I wish I could say you'll overcome this sadness, and maybe you will, but it didn't work out for me.
My grandparents, aunt and dog died in the last three years (my grandpa this month) and it's a struggle for me.
My parents are the only family I have now.
I don't know a solution, the only thing that helps me a bit is focusing on something else, things that you like (music, ATRL, video games, hobbies)! When I do regular things (like at work, or lying in bed before sleeping) all the heavy thoughts are there though