You don't sound like you have too much experience training horses.. Maybe you should get a trainer to come help you, because it could be potentially dangerous training a horse.
Otherwise, just go out every day, 15 minutes a day, and rub her face all over. Stop every time she shows acceptance, then start again. Don't give her treats as a reward, treating a young horse can get them nippy. Reward by stopping whatever you are doing, then start again after around 30 seconds. When touching her face, touch her ears inside and out, open her mouth, play with her tongue, touch over her eyes, on her nose, etc. If you put your hand on her nose and she tries to bump it off, keep it on till she stops, then remove your hand. Keep doing all this until her head is desensitized.
“Have fun - Stay on top.”
"There's nothing like sixteen hands between your legs"