I am going to assume that your heifer is broke to lead already if your show is in 3 weeks, like it was stated above, if she is not you need to get busy.
I believe you are asking how to stop her bad behavior, it does not sound like she is afraid, more like she does not respect you or your space. First, I would start using the gate and only the gate, do not climb the fence. Second, get you a piece of ½ inch PVC Schd 40 about 2 feet long, start carrying it with you when you go into the pen, if she tries to butt you or puts her head down like she is going to charge you, tap her on the nose with the PVC, you do not have to hit her hard, but you do need to tap her hard enough that she feels it. She will start to respect your space, you are not being mean, you are just putting her in her place, it is no different than the alpha cow putting the other cows in their place.
It should not take more than a couple of times to break her of this habit, and you can get on with working her for the shows without too much worry. The longer you let this behavior go on the worse it will get, and more dangerous for you as she grows, you need to put a stop to it now.
Good luck and I think you will be fine once she learns she can't act this way toward you.