I had to go through the exact same situation a couple years ago, OP. My dog just one day stopped eating, and then stopped drinking. He just lay on his bed in front of the fireplace and wouldn’t touch anything, even if I set his bowls right next to the bed or held food to his mouth.
I took him to the vet and they couldn’t tell anything wrong with him other than age - he was a rescue with no prior records, so there was no knowing exactly how old he was, but he was probably around 13 or 14. They gave him a fluid injection and I took him home, but when I got up the next morning, I found he’d thrown it up. My only choice was to let him go.
I had to carry him to the car and into the vet’s office. I was with him in the room, petting and talking to him until he was gone. I had to take the day off work afterward because I was such a wreck.
I had to let another go a couple years before that - my sweet old boy who’d been with me since high school. He was 15 years old at that point, and the breaking point was when his back legs started to fail. He couldn’t stand up on his own anymore, so he just spent all day lying on my bed, and I had to carry him up and down the stairs and hold him up whenever he needed to go outside. I was able to plan ahead in that case, and called one of those vets that comes to your home to do it.
They came in the afternoon, so I took the day off work and spent it lying with him in bed. He was a bigger dog, and he got to lay on our couch with his head in my lap, hearing my voice until he was gone. I carried him out to the vet’s car after that, and realized how much of his weight he’d been leaning into my chest all the times I’d carried him before, because he was so much heavier in my arms now.
The only more heartbreaking experience I’ve ever had in my life was when my father passed. I hope yours pulls through, OP. I really do. I don’t have any words of wisdom or comfort if they don’t. There’s nothing to make you feel better about it. Every moment from when I made the choice until days after it was over, I was second guessing myself if I’d done the right thing, because it’s so hard. It’s so hard making that choice to end the life of someone who loves you, and who you love back, and whose whole world is your responsibility. But you have to make it, because part of that responsibility is knowing when their life is beyond your ability to protect. But it’s not easy. It’s not easy. And it shouldn’t be easy, because we love them so much. And there’s nothing but time that heals that.
Good luck, OP. Good luck.