As much as I agree you should get some help in person (we can't see what's actually happening) - here are some things you can start with for asserting yourself.
Practice teaching her to move away from pressure (a horse that moves away from you is a safe horse). You can do this in numerous ways:
- move her hind end away
- move her front end away
- move her sideways
- move her back
- move her forward
- turn her away from you
How to do each:
With a halter and lead rope on, hold her so she is standing still. Rub the spot you're going to push away from you with your hand, then apply a small amount of pressure with a few fingers (about enough to dent the skin in but not actually pushing). Wait, if she gets distracted or ignores you shake the lead, get her attention back on you, apply a tiny bit more pressure if she really disregards your pressure. The moment she leans away from the pressure, even a tiny bit, stop pushing and rub the spot, telling her she's good. Do this again and again until she moves her feet readily away from the pressure. Eventually you'll be able to point at a part of her and step into her space assertively and she will move away. You want her to willingly move out of your space until you invite her in.
Where to push:
- to move her hind end, apply pressure to the squishy muscly part of her hind leg
- the front end is harder, apply pressure to the squishy muscly part of her front leg, you may need to push a little closer to the point of her shoulder or a little further back, depending on her response. If she walks forward push closer to her neck, if she backs up push closer to her elbow.
- to move her side ways apply pressure right where your foot goes when you're riding, this will help reinforce her giving to leg pressure when riding too.
- to move her back apply pressure to the center of her chest, you can also use the halter, applying pressure to the bridge of her nose, eventually just looking her in the eye and stepping assertively into her space while saying 'back up' will get her to back away.
- move her forward, by leading with the rope, or by driving her away
- you can teach her also to turn her head in either direction by applying gentle pressure to either side of the halter while standing still and waiting for her to turn her head.
- to make her turn away from you, bring the rope around the opposite side of her and either up over her withers (if it doesn't slip up her neck) or all the way around her hind legs (up resting over her hock, don't let her legs get tangled!) and apply pressure to the rope until she turns herself away from you.
All of these skills helps put you in charge, it will also help you feel more confident leading her, knowing she'll respond to your ground cues. They're also very beneficial for when you're riding!
Hope this helps, but please do consider getting a trainer to help you out - it won't hurt. Sometimes people have creative ideas of how to help.
Good luck to you and your horse :)