Tips to correct herd sourness?

A year ago I got a Qh-TB-Arab cross mare who was neglected for four years. She's always been a little herd sour, but it seems to be getting worse. I do groundwork with her at least once every week (normally 2-3 days per week) to assert dominance and let her know that I'm her friend. She is very friendly in the round pen and trusts me (she bows, nickers when I come, join-ups, bridleless) but when you take her out of the round pen and refuses to turn left. She will turn her head but not her body, no matter what cues you give her. Only on her left turns, though. I've had her teeth checked, saddle fitted, and looked at by a chiropractor, but she still acts this way. I ride her in a Mullen mouth Myler bit with 5" shanks and three rollers as well as a Myler comfort D-ring snaffle with a port. I've tried making the pasture a place of work and the arenas a place of rest, but it doesn't seem to be working. Does anybody have any tips to correct her herd sourness?