I'm so sorry - I can imagine how you must be feeling, because my dog is also my other half and I don't know what I'll do the day he passes on.

You need to just keep reminding yourself that they will tell you when the time is right. They don't want to be in constant physical pain, which comes with old age, and you making the decision to let them go of their misery is the bravest, yet kindest thing you can ever do. It is very self-less, as it is painful for you, yet setting her free.

Allow yourself the time to grieve when the time comes. I have a few animals in my life, and I would try distract myself by giving my time, energy and love to them, which is exactly what your beloved pet will want you to do....