Seems like you're not very confident with your mark-making. Usually it's okay to be sloppy at the sketch-level, but when you get to full-colored drawings, it helps to be able to have a certain kind of smoothness to your lines. To achieve this, I'd recommend taking some time out of the next few days to practice drawing straight lines. Just straight lines. Try doing it with your arm, and not your wrist. This doesn't mean that you need to completely switch over to arm-type drawing, just for straighter lines. For curves, feel free to screw around with your wrist. Practice mark-making. Just make different kinds of marks with your pen/pencil. I guarantee you that you'll find a new kind of line control that will aid you in many differing ways, so long as you practice consistently.