She has balls I'll give Willow that. To walk in there like nothing was wrong. She was so sure her spell worked she never considered they were on to her. And that was pure stupidity on her part. They caught her out in the beginning with all the spells she did. What would make her think they wouldn't be on the look out for more? I do disagree slightly though. The hellmouth might have had a bad influence on her magic abilities but I think a lot of it has to do with Willow's personality. Vamp!Willow was brought to this dimension because of her ego and it has just gotten worse. She always hated being shy, geeky Willow and now she has power. Enough power to change that and she likes it. She likes being the one with the power, likes making others dance to her tune like she had to in high school. You get the feeling that her parents only noticed her when she did something well, when she excelled at something. So she treated magic the same way. Only she forgot to learn the rules and just went for the power. I don't think the coven will take too kindly to her attempts at control. Hopefully they can change her. Because if all they do is teach her how to control her abilities that will just make her more dangerous. One mentally unstable witch with even more power? She'll be able to do everything she wants with no one to stop her. 

Tara was right. If the only reason Willow follows the training from the coven is the hope or promise that she'll get Tara back... It would be for all the wrong reasons and when that doesn't happen? She will go nuclear.