When the time comes that they can no longer live a fairly comfortable life we have the responsibility to let them go and end their pain. We wouldn't want a human to suffer needlessly..why would we do the same for our animals?
Know that you have made the right decision and being there with her at the end will ease her into sleep. She will be comforted and loved.
My deepest condolences though...making that decision is extremely difficult but know the pain will ease and the good memories will follow.