So sorry about your old mare... we have a pasture of older mares too, and I don't relish what inevitably will come. Wouldn't trade them for anything though, and I know you must feel the same.
I commend you for ending her suffering when it was needed, as difficult as it is to do so.
We have a 27 year old mare who is beginning to show her age and I dread the day when the decision will have to be made for her. But, when we take responsibility for and love these animals it does unfortunately happen. Having them is very much worth it, though it also hurts so badly at times. My condolences.