I think it's a combination of further developing the relationship between you and her (not only making yourself part of her herd, but you being alpha mare
) and providing variety. Working more at the barn while having more fun time away (think treats and grooming at the end of the pasture instead of at the barn) helps. Don't immediately dismount at the barn either. That's reinforcement for her current behavior.
I just bought a little TWH who showed to be quite barn sour at his previous place. I have a paddock that all new horses go into for a few days before I'll turn them out with the herd. They can touch noses but they're not in the same pasture together. His behavior (constantly calling out to the other horses) tells me just how herd dependent he is. The constant neighing went on for two days. I felt sorry for him, almost put my mare in there with him but then thought "I don't want him bonding with her, I want him bonding with ME!" So I'm spending a lot of time with him and making myself his comfort. He's looking for a friend and I'm it.
Today was a lot better and he started nickering and neighing when he saw me come out to the barn. I'm going to take advantage of this and keep him in there at least through the weekend...if he keeps improving. I'm working him some, asking him to back, yield, etc each day while also just spending some quality time with him. I've never had one this bad before so I'm hoping that this tactic will help jump start working on this issue.