The trick is to let loose and not be afraid of making mistakes (practicing imperfect), as well as focusing hard on getting the details and the overall design right (practicing perfect). When you've mastered both, you can begin to blend -- learn when to let loose, and when to focus hard -- and work that balance.
I am guessing that you are too worried about what the end result will look like. Who cares! Let loose. Enjoy every stroke, every curve, every line, have fun and explore!
Try even things (styles/media) you've never tried before! B) That often helps with loosening up. What you want to avoid is staying in your comfort zone. Dare to experiment and to make mistakes, boldly.