Be the boss. Many new to or getting into horses as adults folks are worried about hurting their horse or being too mean. You need to realize your mare is a 1000# several foot tall critter with some thick skin and she could crush you if she wanted to. If you do not command respect from your mare as soon as possible, you will be fighting for it for the rest of your relationship.
Not knowing exactly what she's doing, it's hard to give proper advice. If she's crowding you, carry a crop and give her a tap increasing to a swat if she gets too close. If she's trying to rush, a tap to a swat on her chest will tell her to stay with you. If you have someone to help and instruct, you might want to try a chain over her nose, wrapped around the halter's nose strap. Repetition and consistency are going to be key--you can do it!