While it's tough to see a horse you love go, (esp if you were gone when it left and didn't get to say goodbye) think about the big picture for this horse. She was rescued from a bad situation, she was brought in and lovingly taken care of, and now she has a home. That's what a rescue does. It sounds like you did your part well so be proud of the hard work you put in, grateful for the love you were able to give her, and happy she now has a home that will love her. There's another horse out there that is needing the same love and attention you gave this previous horse. How would I cope? Take another day or two to mourn if you need it (yes, you can mourn the loss of a horse that didn't die), and the move on to the next that needs you.