I think it is time to say your goodbyes, give him as many carrots as he'll eat, let him know how much you love him, thank him for all he has done for you, and then give him the greatest gift of all - allow him to slip away, pain free before his suffering grows.
I put my beloved Hugo down early this year, he was my pride and joy, I would still do anything to have him back, healthy. But after numerous, ongoing injuries - including arthritis in his hock (thanks racing, Hugo was only 10), he then slipped in the paddock and tore his suspensory ligament, and two weeks into stall rest, with his leg in a cast, he still managed to bow his tendon in the same leg.
I could have kept him in a stall, for months on end. But weighing it up, he was unhappy being confined, his hock was starting to swell again and he was starting to have trouble with the oposing foreleg.
It wasn't fair to keep him alive, for the selfish reason of me not being able to let go.
We must always bare in mind, that a horse does not have a concept of the future, nor the fear of death that human's have. He does not worry about what may happen in the future, he lives in the now. And if the now is uncomfortable, stressful, or painful, he is not going to be happy nor healthy. Putting him down will not hurt, it is over in an instant, he has no idea. He doesn't think that you are letting him down. If anything, he will probably be thanking you for easing his suffering, and providing with a long and healthy life.
It brings tears to my eyes to see ancient horses, riddled with arthritis, unable to gain weight and clearly sore and simply over it, standing around in paddocks or stables until they become some weak that they colic or die in an otherwise unsavoury manner. Horses don't tend to just pass quietly of old age, they will simply become so emaciated that eventually they will colic or bleed internally.
I could not put a horse through that, and will willingly lay my horses to rest well before they get to that point.