First I would check the area surrounding the paddock gate on both sides to see if there is any kind of object that may have scared or hurt her. If nothing there try taking her towards it at several different angles to see if that helps. My sister used to have a TB that wouldn’t go on the wash rack in a certain direction but if we went the opposite direction she had no issue.
Second ask to see if the same person has been taking her to the same paddock, if so see if having someone else takes her to see solves the problem. I dont know anything bout your barn or the people that work there but the only reason I can think of as to why their would be a problem with a certain person in a certain area is that did something in that area. Not saying thats what happened but its possible. or it may have been an accident. IE: your mare spooked at something in that area and they reacted and jerked her lead and/or beat her.
If having someone else take her in doesn’t work try whats suggested above or another option is to use the reward system (use her favorite treat and over a period of time work your way from right before she starts to act up to where your several yards past the gate. Have her stop for a minute or two and if she doesnt “spaz out” give her the treat then move another few feet forward and repeat. Keep doing that until your able to get her to your target area. The amount of time it will take will depend on how big a stretch land and how far you want to push her each day. If your able to get her to go only a few feet past her comfort zone each day before she starts acting up at least it a few feet closer.)