I wouldn't discount physical issues - be good to get her checked out, especially if she's only 3yo & worked & not living naturally - some good reasons for physical probs to be more likely.
But I am thinking mental/emotional stress. I wouldn't play that down any less either, because aside from it not being nice for her to always be cooped up alone, her behaviour could not only cause obvious physical injuries, but - I think even more importantly - stress causes other physical probs(gut, metabolic, disease, etc). I may have it wrong, but it sounds like she's stalled 24/7, only gets out on average every other day & that's into an arena or 'work' without company of other horses. I would drastically change this management & keep her in 24/7 turnout with a herd.
She has been checked out for physical health at the beginning of this issue and checked out at a healthy normal 2.5 year old at the time.
She is not stalled 24/7. She has a huge corral attached to a barn that she has free choice of in and out of the stall. There are two other mares that share that barn but not close enough to touch noses. She did get turned out daily into the arena but she started busting the fencing and that's become costly.
Her 'getting worked', I should have been more specific. The work is a bit of lunging, some pressure and release games and detailed and fine tuning on ground manners. I ride her maybe once a week for maybe 10-15 minutes mostly at a walk very little at a trot. I spend a lot of time with her just hanging out too.
The majority of my life I've had only one horse at a time and never had them become lonely like this or act out like this. If that's what she is is lonely.
Boarding, I don't have much of the choice to turn her out with the other horses unfortunately. I have seriously started to think about getting her a companion, possibly a miniature donkey?