A couple things I think are important - a rhythm, pre shot routine and being relaxed. Perform a mental checklist of these and remind yourself about them throughout the match to be sure you are consistent with them throughout. When I am under pressure, I tend to blow one of these three. For me, I think it is the "stay relaxed" part. I might say same for you if you say you tend to steer in these situations. In your pre shot routine, as you are standing back from the shot, perform your practice strokes - nice and smooth and fluid. Visualize a smooth perfect stroke through the cueball. Remember to breath. A nice easy exhale, then step into your shot. Same rhythm and cadence as the rest of the match. If, when you are down on your shot, you start thinking too much, feel yourself tighten up, etc., get up, back away from the shot and start your pre shot routine again.
- Steve