Is it such severe lameness that pain killers like bute, Previcox, Banamine, etc don’t even touch it? Cold hosing/icing doesn’t reduce swelling/pounding pulses?
There should be things you can do to ease discomfort while the horse heals. If nothing is working, the lameness/injury is so severe it may be time to consider other options like humane euthanasia or at the very least serious pain control. I worked at a barn where a horse was on fentanyl patches to control pain. There are extreme options if the horse is that uncomfortable.
A horse that will be in chronic pain for the rest of their life is not living, they are suffering, in my opinion. But I am not clear as to if what you are speaking about is an injury with a decent prognosis or not. Some discomfort during healing a soft tissue injury, a wound that required stitches, recover from surgery, etc is to be expected and as long as there is appropriate pain control you are doing what any good animal owner would do. If the horse is never going to improve and the state they are in is likely their state for life, it is time to consider all the options. You can’t dope up a horse on painkillers for the rest of their life to make them some semblance of sound.
Relief can come from knowing, for sure, that they are not in pain anymore. And I say that knowing how hard it could be to consider putting down a horse you love.