Well, several folks have indicated that this could very well be a medical issue - I wonder the same thing.  Have you had her looked at by a vet?  It would be great to find out that it's something "fixable."
ETA:  She also could just not trust humans; IMO, poking her with a sharp stick isn't a good way to teach her anything.  They learn from positive reinforcement best.  I had a mare that was quite neglected by her former owner; she really had no reason to trust humans.  I'd put her in my round pen, go in and set down on a bucket for HOURS at a time, just sitting there.  Finally, after many days, she came up to me on her own, and I offered her treats.  It was a long process, but by the end of that first summer, she knew her name, wouldn't pin her ears, and stopped biting (yep, she was pretty nasty when we first got her).  I never even attempted to get on her until we got to the point where she could trust me on the ground.  Now, 6 years later, she's absolutely the sweetest horse I've ever had!!  
Good luck, regardless of what you decide!