Aww :( I'm sorry it came down to this. I was in this same situation about three years ago. I had graduated high school and was getting ready for the big move to college. My Saddlebred mare was about 23 as well and I knew I couldn't bring her with me to school. Putting her up for sale was one of the hardest things I ever did because I felt like a horrible owner putting her up for sale. :(
HAPPILY, though, a wonderful lady contacted me and was interested in taking her! I was so glad to have found someone who wasn't just going to sell her to the slaughter house! I even let her have my mare for free because I was just so relieved, plus she had traveled two states to get here. She was a wonderful lady who kept me updated on her and even sent me pictures. :) She retired her until the day she died at 25.
I'm very happy for you that she is going to a good home! :) It saves a lot of worrying and sadness.