To have your thoughts be unimpeded means — to some extent — removing the obstacles that impede them. Some of those obstacles are doubt and second guessing. Another obstacle is lack of skill. The latter obstacle has all kinds of irony attached to it since going forward with one’s desire leads eventually toward gaining the skill. If you draw when you think that perhaps you can’t, you move yourself in the direction of getting the skill that you fear you lack.
It is self correcting. So you just have to draw anyway. Draw as though you have the skill, as though you have no doubts, and you have removed some of the obstacles that impede thought.