How can I teach my horse to not be so herd bound / barn sour

In short it will take some time to get your horse to be confident in themselves and to have that feeling of "been there, done that" attitude. What you can do is ride your horse a lot around the barn and then when you are done take your horse out for a walk while she is still wearing her tack and saddle out on the road and allow her to graze, explore things, and relax. If she doesn't relax then you can get tense and excited for 3 seconds and then you relax and picture in your mind being very tired and sleepy. If she continues to be excitable then just stop and pet her at her withers till she calms down. Then take a few steps back to the barn and just stand there and hang out till she grazes quietly. Once she does walk slowly back to the barn and call it a day. Practice this for several days with each days goal of trying to go out just 10 yards farther than the previous day. Once you notice her being calm no matter what distance you go then you can practice this while being mounted on her. The key is the petting at the withers so that she associates that feel of your hand as to being a calm down cue. This also mimics in a way the feeling the foal gets when it stands underneath it's mother.