Deal breaker for me as well. I have one here and she will be the last. She is not manageable, we've tried every option over the years aside from a shock collar and at 25 years old it's too much of an ingrained habit to try now. She is a fantastic mare otherwise and the best babysitter I've got but she is destructive. She trashes fence posts, stall doors (on the rare occasion she is in for an hour, she has a party) and has ruined her teeth. I love her dearly, have been riding her for 20 years but once she is gone I won't ever have another one on my farm.
Life is like a camera. Focus on what's important, Capture the good times, Develop from the negatives and if things don't work out, Take another shot.