OP, I'm new to horses too. Probably more new than you as I've only started lessons recently. What these people are saying sounds just like what my coach tells me.
Basically, horses will try to be the boss of you (not do what their told, do what they want and not what you want, ect) if you do not teach them that you are in charge. You have to call the shots 24/7. If they get grumpy and start acting different you just have to be strong right back at them and let them know that this behavior is not acceptable.
For example, one day she's easy to catch, the next day she runs. The day that she runs do not give up! Stay after her until you catch her. You could then spend a whole day training her on catching. She should know that if she does something unacceptable that she will have to do it the right way and repeat that action the right way over and over. She will eventually learn that you mean what you say and she might as well do it right the first time.
She is testing you. Do those things over and over again so that she knows that when you tell her to do something you mean it. And like a little kid she might push you and test you to see if you really mean it. You just have to stick with it. Just like a toddler you have to constantly reinforce good behavior and let them know who's boss when they try to do something they shouldn't. It's all about manners.