A.V.
You are making a smart, if heartbreaking decision. My aunt put down her beloved doberman when the dog started snapping at children. Nobody got bitten, but the dog was unhappy, was no longer able to get up and down the stairs and hid under the table all the time. She was in pain and her pain made her irritable. It was far kinder to say, "I'm sorry, old girl. But you won't suffer anymore." I am sorry you have to make this call. I've been there myself and it's hard. But sometimes it's in the end the kindest thing you can do. I have only regretted waiting and allowing the animal to be in more pain because of my indecision. I have not regretted letting go so the pet no longer suffers.
Big hugs. Remember the good times.