I just happened to take a lesson and they showed me a great drill to test how straight you stroke
at higher speeds (well above lag speed). This will make it clear how bad you steer and
you can try different fixes and immediately see the results.
The 1 and 2 are exactly 2 ball widths apart (so just line up 4 balls and remove the middle pair).
Shoot hard enough to pass through the gap, back up to the head rail, then back down the table
and through the gap AGAIN.
At this hard speed you stroke flaws will really show up. I often hit one of those balls
immediately after bouncing off the first rail.
You will need to figure out a fix that works for you, such as holding the arm further from your body,
or closer, tucking in your elbow, or cocking your wrist inward a bit.