I haven’t had to put a horse down yet (thankfully), but I did go through a similar situation with my cat who I’d had since I was in middle school back in the spring after he developed a pretty aggressive form of liver cancer, and all I can tell you is to listen to your gut. I could’ve thrown money at my cat and tried to treat it, but he wasn’t eating and he couldn’t get up and down the stairs to where the litter box was and I knew that it was just kinder to let him go. It’s the hardest thing I’ve ever done in my life (this might sound ridiculous, but there was a lot of mental health recovery wrapped up in that cat), but it wasn’t fair to ask him to continue suffering just so that I could have more time with him.
My vet left the decision up to me, but she told me afterward that she thought it was the right one, and also reminded me that, while it’s the hardest part of being a pet owner, the kindest thing that we can possibly do for and greatest gift that we can possibly give to our animals is to be willing to let them go when the time comes, even if we aren’t necessarily ready yet. The only guilt I feel regarding my cat is that I was incredibly busy with work when everything happened and I still can’t help but wonder if I would’ve noticed sooner had I been around more, and either been able to do something to help him or had more time to come to terms with all of it (we figured out it was cancer on April 1st and he was gone by the end of the day on April 3rd, so I had no time at all). I don’t regret letting him go when I did. He gave me a great gift over the course of his life and the least I could do for him when he was no longer comfortable was let him rest.
From the sounds of things, your horse is having more bad days than good ones, and, while I can only speak for myself, to me that means it’s time. It’s not wrong to let them go when they aren’t comfortable anymore. It’s a kindness. Sending you lots of hugs and support as you make this decision. I know it’s not an easy one.
