I am so sorry for your loss. It will be two years this spring when the SAME EXACT thing happened to me. Its a hard decision to let your mare go. When I lost my mare, of course it was a filly that she was careing and the most the unique markings the filly had. She had a beautiful bay colored head, and bay color on one side, and the other side was totally white, with a black mane and tail. She was long legged and long refined neck. We had tried for hours to get that baby out but no luck. I had to leave the barn when I made the decision to let her go. I was 20 at the time and I couldn't bare to see a mare go who was eating contently while laying there.(she was also my first mare that I got when I was 16)So my mom stayed her, with her head in my moms lap.
~~~Again sorry for your loss.