@summerfield - no matter how much you have done to try to make this horse comfortable, no matter how much time and money you have spent trying to get and keep him sound there will always be people asking have you tried this or that or consulted this vet, etc. Always.
You are done when you realize that all the things you tried, all the time and money you spent have just won short respites of some degree of soundness. You are done when your budget for chasing this wild goose has run out (an amount set by YOUR budget). You are done when you are unwilling to chase that wild goose any more. You are done when you can’t keep looking at your horse in pain or drugged knowing it’s never going to get any better than this.
All of these things are personal limits. No one can tell you how much money and time you must spend. No one can tell you how much pain your horse must suffer.
And that’s okay.
But it is still going to be hard. You’re still going to second guess yourself. It’s natural when you love the horse. Just remember that you love the horse, and you know him best. You can see the new shoeing has won another mostly sound respite. Mostly sound.
I have euthanized two of my horses. The first one gave me the gift of confidence to trust myself. Trust that I did know him. The second gave me the gift of the right question. Not “Can I” but “Should I keep him going?” Winter is a big deal here so for me the question was about weighing the risks of keeping him through another winter or not.
I believe in letting them go before the bad days outnumber the good. While they are still enjoying going out with buddies, snoozing in the sun, etc. If I were you I would say thank you for this respite of mostly sound, enjoy him a last few weeks, and let him go before it gets worse again.
Trust yourself. Accept that it is going to be hard and is going to hurt emotionally. And trust yourself.
(((Hugs))) 