Yup, just keep exposing her to them. But don't force her all at once. Take gradual steps.
Let's say she is comfortable walking by the cows if she is 300 feet from them. But she is uncomfortable and freaking out if she is 299 feet from them. Don't ask her to face the cow per se, but ask her to do manuevars that move her back and forth between her comfort "safe" zone that the zone where she is too close to the cows (from her viewpoint). Ask her to do walk/trot transitions, backing up, sidepassing, rollbacks, etc. Just anything to keep her focusing on YOU and doing what you are asking her to do.
Pretty soon she'll be walking comfortably when she is 299 feet from the cows because you've kept her mind busy on you and not the cows. Then start working at 298 feet close, the same thing.
Also, focus more at walking past the cows, rather than straight at them. I find this works best for most anything a horse is scared of. By asking them to just walk past the thing in question (and making them do manuevars as you go) it takes their attention off the scary object or scary cow and before you know it, they are walking right by it back and forth and then the horse realizes that there's nothing to be scared of too, now that they are that close and nothing has happened.
It will just take time. Go easy with her and just keep her in between that comfort zone and the " i'm freaking out" zone. And keep her focused and listening to you the whole time.
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It is not enough to know how to ride; one must know how to fall.