I went through the same thing with my first mare, Summer. I had outgrown her in terms of ability, and I knew she no longer fit in my string, but I cried and cried when I finally sold her. After she was gone, I occupied myself with my other horses and got over it much sooner than I expected... of course it helps that I Facebook friended the new owner (recommend!) and get updates. Plus, it was comforting to know that she was working at a level that was more comfortable for her. If you do your research beforehand, I think you'll find the process much easier than you expect.