You need to start over with this horse from the ground up. If you dont have acess to a round pen, you should try fencing off part of the field she lives in, it doesn't even need to be an expensive fence, you could even fence it off and make a sort of round pen with green T posts and white tape, etc. You need to start from the ground up, as well as have a thorough physical exam done by a vet. MANY problems with horses under tack and while ridden are due to pain, some that can be hidden really well. She could have a sore back, malalignment, a problem in her legs, a problem with her teeth or ears, or neck. If it's not physical, it sounds like a lack of respect for you and lack of trust in you to be the boss. She needs to learn this on the ground and then in the saddle. Try looking at programs like Parelli, John Lyons, you can do these on your own with your horse and correct a lot of behavioral problems, as well as teach her she can trust you. Because she didn't have this problem when you got her, it could be any combination of things. You need to start from the basics of ground work and teach her to trust you and try again. Don't give up on her and admit defeat, you probably just need some extra help and honestly those programs and "games" do a lot to foster the relationship needed for a successful partnership. Don't give up, just start over from scratch. Good luck.