It is quite difficult, for two main reasons. One is that the pencil is quite large, making some collisions hard to avoid. The other is that the controls are a little hard to handle, since there is rotation with the mouse and WASD/arrows to move forward and backward. From a game design perspective, though, making the pencil smaller might mitigate the charm of the barrier drawing, since all of a sudden you don't have as big a radius. Maybe there is still room to make it smaller though?

Another thing is that the shapes start to fly around the screen much faster than I can react to them.

This is only my experience with the game, and my view on the difficulty could absolutely be my limited skill. Don't forget that you have made quite an original look and play. 

Thank you and sorry for the late response!