When taking a practise swing, most everybody is nice a loose, relaxed, because there is no expectation of hitting a ball, there is no thought of aim, power, contact, the list goes on, basically your mine is clear and relaxed. Then when go to hit, tension creeps in, you begin to get target oriented, you want to hit the ball. This problem plagues every one at some point, you see it with the pros too, just not as dramatically, some call it choking..lol

My only suggestion is when you do go to hit, try to swing with complete abandan, don't worry about where the ball is going, swing relaxed, don't tense, hard to do, but with time gets easier, just remember you already took care of the aim at setup, your grip will take care of face alignment, your body knows how to swing from practise, so the only thing to screw this up is you trying to help and force it.

Watch the ball after you hit, it will tell you what you did wrong, then use the practise swing make the adjustment to correct, when you step upto the ball, don't be thinking about the adjustment just setup correctly and go, the longer you think about it, the more tension set in.