The horse may do exactly what you tell her to do when it's something she is comfortable with. You haven't yet gained her respect, and therefore she sees no reason to listen to you when you ask her to do something outside of her comfort zone. That is the issue in a nutshell. You have not had her long enough, or worked with her enough to gain her respect as herd leader, apparently. It can often take mares longer to get to that point than geldings. Especially if she has lead mare tendencies. Get the respect first and then work on the water issue.
One thing that really gets my hackles up, and this isn't directed at you OP, except that your one comment brought up the topic, is the "jump on the abuse bandwagon" state of mind that seems so common these days. SO many people, (I see it here on the forum almost daily) the second they have an issue, want to blame it on "maybe Fluffy was abused and that's why she doesn't like so and so". 99% of the time, Fluffy just doesn't respect you enough to behave or like whatever you are asking. It has nothing to do with abuse. Horses have minds of their own. They do not have to suffer abuse in order to dislike something.