Ok – first things first. Fistulous withers are often caused by exposure to cattle with brucellosis. If your horse is still around those cattle, she should be moved. It can also be caused by traumatic injury, so you might want to check your tack for correct fit.
Go on you tube and find some videos on saddle fit, or visit your local library for books/videos.
This poor mare sounds like she’s had a rough time, probably before you purchased her. Please have the vet give her a thorough vet check, like a purchase check, before you start her under saddle again.
You have to ask yourself, why are you not confident? What can you learn to make yourself more confident. Go off the farm for lessons on another horse, with a good instructor, to learn more so you don’t feel that you will ruin your horse.
Books and videos can be helpful. Buck Brannaman has given me a lot of help in ground work and handling, his philosophy makes sense to me. It’s not the methods so much as the ideas. Other natural horsemanship people, maybe not so much. But any good trainer knows these things and practices them.
Horses are herd animals and like to have a leader. That’s you. Keep her out of your space. She moves her feet, you don’t move yours. When you ask her to move out of your space, she should do it. Don’t let her move into your space and push you out of the way. That’s kind of it in a nutshell.
if the horse is trained, as you say, then she knows these things and you have to reinforce it. There’s no need for violent force, but confidence is essential.
Once you feel you can move her around at your will, your confidence will increase. Get to know how horses think, and then you won’t be as confused. Pop culture trainers won’t help, but real horsemen will.
Start slowly. Move her around when you are grooming. Require her to lead next to you, without going ahead or lagging behind. Pick up her feet. Stand still as long as you want her to. Things like that help a great deal.