My horse often pushes me and steps on my feet when I try to lead her especially when she is around other horses and is excited. How can I get her to stop doing this? You've already figured out why your horse is doing this and that is because she is excited and nervous. What you can do is show her that you are a strong leader and that she will be better off by remaining calm with you.
So for starters make sure she moves her hips away from you at your request from the ground by either you touching her side or better yet just by glaring at them. You can use a spinning rope too to get her to move over.
After she knows that well then you can teach her to respect your space by if she starts to get to close to you, you end up flapping your arms like a chicken and march with high knees. If she gets too close she ends up being bonked by your arms and knees. Once she moves away from you keep flapping for another 5 seconds but don't try to hit her. Just act like you are a flapping machine that she accidentally ran into.
When she knows to respect your space at a stand still then go ahead and lead her. If she starts walking on top of you do the flappy thing till she backs away. If she just starts moving past you then you turn around 180 degrees and walk back to where you were coming from. Then make a gradual turn towards where you want to go. After a few times of realizing that if she stays behind you or to your side and not on top of you then she won't get bonked or have to make sudden changes in direction. If she decides to ignore you because things are just too exciting then you can have her do small circles by moving her hips over like I explained earlier. That little thing just becomes a reminder that you can control her feet even when she is excited.