well, you can either let nature take it's course or take over yourself. If you are not able to bring yourself to end it then let nature take it's course. If she's unable to swim and isn't eating then nature will do it's bit.
If you decide to put her to sleep clove oil is kindest. Here's what to do - Arm yourself with a 500ml pop bottle and wash it out. Add to it about 25-30 drops of clove oil and fill up with tap water. Shake really well.
Get another vessel (make this something you can throw away. Clove oil stinks and it can not come into contact with your tank either) and add a little tank water and poorly fish. You can then pour in the water and clove oil mix, don't do it slowly do it quite quickly. The fish will stop breathing within about 30 seconds, sometimes faster if they are very weak. Leave fish in the mixture for an hour to make certain it's not woken up then you can dispose of fish and mixture.
As I said. Do not allow clove oil to come into contact with anything that is likely to go into your tank. Keep things seperate (I use a 2 litre bottle that I cut in half and keep it under my kitchen sink) It's also a good idea to wash your hands thoroughly afterwards