Where you are at now is, I believe, the worst time. You feel awful, but once you have actually sold the horse and they have gone to their new owner, it all seems so much better (if you have picked the home you prefer for them).
Remember, the person getting the horse is as excited as you were when you got it, and despite what we like to believe, and as you say, often the next person looks after our horse better (at least as well) as we do.
It'll be fine, you will feel much better once the deal is done and later down the track if you decide to move horses up the priority list again, you will find the perfect horse for you then. You can't always fit horses in on your life journey (I dropped out of horses for 15 years and then went back into them).
What is best for your little horse, that is what counts.
Only As Old As You Feel - Sometimes I Feel VERY Old