You show her that you're the boss by actually being the boss. It's not so much about whether you can push her around or make her do things. That's how you show dominance and it's not difficult to do with most horses as they're naturally easily intimidated by a human being. Dominance plays a part in it but what a horse trusts is a person who makes good decisions, keeps them out of trouble, is consistently cool-headed, isn't easily startled or upset by anything that happens, provides a calm and reassuring presence when the horse gets unsure or scared, and builds their confidence by challenging them to get better without burning them out or over-exposing them. These are things that only come with time, study, sacrifice, experience, and an unquenchable desire to reach past the mediocre and strive for excellence in your horsemanship.