Three years ago I got a four month old filly for my daughter to train. Like your horse, my filly had never had anyone touch her. She was wild and scared when she arrived at my place. I put her in a pasture with an older mare who immediately bonded with her and got her over being weaned from her mother.
Then I built two pens inside the pasture about 20 ft. by 30 ft. each, and put her food and her buddy mare's food in those pens. I fed the horses twice a day in the pens, letting them out when they had finished eating. After the filly was used to eating in the pen, as Yogiwick suggested, my daughter spent a lot of time just being in the pen with the filly. It wasn't long before we put a halter on her . . . and took it off. Soon the filly had no problem with the halter going on and off her head. My daughter spent a lot of time grooming the filly in her pen. Then we began teaching her tricks, taking her for walks, letting her eat in the trailer, taking her out to graze, although she had all the grass she wanted with her buddy in her pasture, she got used to being away from her buddy and the other horses.
Then we taught her to stand tied, to move away from pressure, to let her feet be handled, back up, and learn general horse handling. She learned to be driven in long lines, had a bareback pad cinched up on her back, and later, learned to stand quietly in crossties.
When she was about 2, we began trailering her with different horses to interesting places like barrel racing and trail rides. She learned to be led with someone riding another horse.
When she turned 3, my daughter began to sit on her, both bareback and with a saddle. As she got used to that, I began to lead her with my daughter sitting on her, and as she got used to that, I began to lead her riding another horse.
Now she is 3 1/2 and is doing great. I rode her along the roadside this morning with traffic whizzing past her. You just have to take everything step by step. Don't ask the horse to make any big changes or assume the horse will know something like "walk forward when its sides are squeezed". Everything has to be taught, firmly but fairly. My daughter now has a terrific filly and the pride of knowing that she raised and trained her herself.