I'm sorry to hear about your fish
The method I use when I end up having to euthanize a fish is to fill a container with ice & water, stick it in the freezer for a few minutes, then once it's cold enough scoop out the ice and quickly place the fish in the water. Death is nearly instantaneous.
IMO, the quick shock seems more humane than slowly freezing to death.
Other methods commonly considered to be humane are placing the fish in a glass of alcohol, the theory being that it acts to numb the fish while it dies in a euphoric state. I don't put much faith into it myself. Then there's cutting it's head off. I simply can't bring myself to use this method, no matter how humane it may actually be.
The worst thing you could ever do to a fish, is flush it down the toilet. If you think about where it ends up, still struggling for survival, you understand that it is about as far from humane as you can get.