I had a similar experience with my younger sister's horse that I had to finish training and sell. I really didn't care for the mare, we just didn't mesh, but I learned so much from the experience. By sticking through it, I gained so much more patience through teaching her to collect and canter, even though she had the worst attitude.
If you feel like you can't maturely finish this horse and that you think your attitude/emotions would get in the way, then I would sell her. If you think you still have patience and are willing to try your best and look at this as a learning experience, then I think it would be very beneficial. You're not going to love and enjoy every part of life, and learning/knowing how to deal with things that aren't always fun is part of maturing.