stop steering: Practicing with only your right arm will help you forget about mechanics and instead putt by feel.
Miss the hole enough times, and you'll find yourself putting defensively. Your grip will tighten, your stroke will become short and choppy, and your hand-eye coordination will get dull. Your putting will go from bad to worse.
That's when you should head straight to the practice green and start putting balls with your right arm only. This drill will help you in two ways. First, it will teach you to release the putterhead properly, which is one of the first things to break down when you're missing putts. Why? Because you start trying to steer the putterhead on the correct path, rather than stroking the putt.
The second thing this drill does is restore your hand-eye coordination. By using only one hand, you tend to forget about the mechanics of putting and simply stroke it to the hole. The best putter under pressure--especially under 15 feet--was Jack Nicklaus. He used to say he would think his right palm was traveling down his intended line.