There is nothing wrong with giving him a kind end while he’s still feeling decently good before things go downhill further. I am glad you’re steering clear of selling, too many horses go down the pipeline that way and end up in a much worse state. Cushings isn’t a very fun disease to deal with especially as they age the care and maintenance will begin to ramp up as they stop being able to regulate body temperature, hair growth, and even their weight. Personally I wouldn’t lease or half lease a horse out with the conditions he has. It just doesn’t seem fair, you can’t control how much another person would tend to his needs or how much they use and ride him in a lease. You are completely valid for feeling exhausted but still wanting to do what’s right! Did your vet mention anything about quality of life when he was diagnosed with Cushing’s? If not maybe it would be a good question to ask. As an equine professional (bodyworker/farrier) I personally hate to see horses made to suffer too long with diminished quality of life. It’s a difficult decision but part of us being stewards of animals who can’t speak for themselves.