I'm sooooo sorry for you...I know exactly how you feel, my black molly Billy (rest him) was really poorly too and I had to kill him to put him out of the pain. I filled a large tub of water and put him in it, then put it in the freezer...a lot of people don't agree with that method, but he swam around for a bit, then slowly got slower and then gradually went unconscious. :'(
From the discussions that people were having on here people seem to believe that it's when the fish come in contact with Ice that they feel pain, so I made sure to be careful that the water didn't freeze...
I was the same and couldn't face spiking him, or bashing him, so I decided that this was the best thing to do...after all, they lower the temperature of humans until their heart stops or slows sufficiently in order that they can work on their eart, so I feel that if it;s good enough for us, then it's good enough for him.
If you need any more advice let me know...is horrible I know, good luck!