You feel like you can mentally control your body because you can give a mental signal to lift your hand and the hand will lift. You can stick out your tongue or shake your booty whenever you choose. Giving signals for the body to do something specific works for simple movements but something else happens when you engage in complex movements. The movements are too complicated for the conscious brain to handle and it has to get out of the way and let the body take over. Even standing or walking is too complex to be totally controlled consciously. When learning to do skills or "difficult" movements the faster your brain stops trying to manage all the delicate movements involved, the faster the skill can be learned.
The sensation of "trying" to do something is usually wanting to learn something faster or control an outcome with the conscious brain. This results in clumsy jerky movements because the wrong part of you is leading the movement. The body itself should be left in charge.
Another way that the body is influenced is by mental state. If you are relaxed, the body will relax. If you are nervous or angry, the body and face will show it. The body is constantly reflecting emotion and mental state. This is natural.
As I have mentioned before, problems occur when the brain fights the body when it is trying to reflect emotion and mental state. People try to lock down their face to hide anger. They clamp down on the body to keep it from showing nervousness. Tension results. If it happens often enough, people can locked in a tense movement or in an unproductive mental state.
When you are doing an activity like wing chun or playing piano, try relaxing and putting all your attention on the activity, but "let" your body do it. Have no concern about outcome or doing it "right" and see what the body wants to do.
Letting go is not easy, but it is extremely important. Grace is what results when the body is in charge.