I am not as experienced as many who post here on COTH, but I think my opinion has some merit since I have four Thoroughbred mares, three of which are Off Track Thoroughbreds (OTTBs).
The first thing I would look for in this horse is PAIN. A good vet can really help with this, but some horses can have problems which are really difficult to diagnose. Have you ever hurt your back? It’s awful, and not all back ailments in humans show up definitively on MRIs or Xrays. Imagine how awful for a horse to have someone sitting on its back if it has back pain, neck pain or pelvic pain. Tooth pain, damage to a facial nerve, or even broken bones in the face or jaw can cause serious problems and are sometimes hard to find.
There is a long list of known problems which cause such pain, like the aforementioned kissing spines. Some ailments, like brain tumors, are often only found during a necropsy. Pelvic fractures are not uncommon in OTTBs. Mares can have not only hormonal problems which can cause increased aggression, but also pain from ovarian or uterine cysts. Many here have written about problems with their horse’s behavior caused by ulcers. I haven’t even touched on diseases like Lyme, and the list is long for all of the things which can go wrong with the legs and feet.
Someone wrote on this forum about a horse they had which went crazy when confined to a stall and once given 24/7 turnout became a new (sweet) creature. So there are mental and social issues which can affect a horse.
The most disturbing thing, and what I see as the bigger problem is that this mare has learned to handle her problems, whether they are pain-related or behavioral, with serious aggression towards you and your family. Personally, I would always worry that even if the physical problem were fixed and her behavior improved, she would still have that capacity to act out aggressively.
If she were my horse, I would have her examined thoroughly by a vet, because I would want to know what the problem was, and if I weren’t convinced that she could be fixed, both health-wise and behavior-wise, I would have her euthanized. What I would NOT do is send her to a trainer who would work her hard when she may have pain issues, and I would not pass her off to another person. Unless she is incredibly lucky, she will go downhill with each owner and likely end up on a meat truck to Mexico.
I’m so sorry you are having this issue with your first horse.