You did what was right for your horse, you put her needs before your emotional attachment. That is what any good horse owner could hope to do. I am sorry that as a horse owner, the circumstances put you in this position to have to make this difficult decision, but you did what was right for Mooney.
Hopefully if anyone out there faces a similar situation, they will remember your experience and will help them make the responsible choice.
And while you don't want anyone to feel sorry for you, I will still offer my condolences out of respect to your relationship with your horse.
Be the kind of woman that when your feet hit the floor each morning the devil says, "Oh crap, she's up!".