Being a mare has nothing to do with it. I've had geldings that have done the same thing. So basically she showed a bit as 3-4 y/o and then didn't really leave home for several years? You need to treat her like she's a baby again and take her to as many places as you can. Not necessarily in a show environment where you might be nervous or there's any pressure to perform. I'd take her to shows just to warm up and hang out by the trailer. Take her on trail rides. Take her to school at other barns if you can. The more she travels the more she'll realize a show is just another place to be ridden. If she is terrified I'm not opposed to giving a tube of BKalm or Quietex at first just to take the edge off (assuming she isn't showing). It'll give her a chance to think clearly as she learns about being in a new environment.