If in doubt don't do it. Once done you can't undo it so you need to be 100% sure in your mind that she is ready. In the past when I have had to do what you are considering it was more of a relief because things had got to a point when the horse wanted to go. Why not wait until it feels completely the right time that way you won't have these grave misgivings now and after the event.
She was a good age and she'd had a wonderful life with loving and careing owners. That's just what I hope I can give my girl if I work very hard. So well done you OP, you've done a wonderful job and she would of known how much she was loved.
The right decision made at the right time. I'm so sorry that you have lost your lovely mare, and I'm glad you're at peace with the decision you made. You've done the last act of a caring owner in ensuring your horse isn't suffering and lives their last days with love and kindness - if only more did the same. My thoughts are with you, I know how you must be feeling right now. x