I dont mean to sound insensitive but.... As much as I like owning fish, I like fishing at a lake. When Im done fishing, I take the fish home and they're dead from being left out of the water. I know you all are going to hate me for this but IMO, if your fish is THAT bad and incurable, AND is dying a slow, possibly painful death, then I suggest you take it out of the tank, put it in a black plastic bag, put the bag on the ground, walk away.... Then come back 30 mins later and throw the bag away.
The fish will die in a few mins after being left out of the water. Even if you put it in the freezer, chances are the fish will die because of no water quicker than it would freeze to death. Look, dont get me wrong; I love all animals BUT sometimes you gotta do what you gotta do. I know its hard since you've grown attached to it.
Think of it this way... would your fish rather wait days in pain and stress until its dead or just suffer for a few more minutes and die being left out of the water?
Good luck.