You start with ball in hand, get position on 1 off of pocketing the 8. Respot 8 on button before shooting 1. Shoot 1, get position on the 8, pocket 8 getting position on 2. Respot 8 before pocketing 2. Repeat through 6 ball. 1 through 6 are evenly spaced on head string with the 1 and 6 about a 1/2 diamond out from the side rail.
I suppose you can make any rules you wish but the drill would be designed to have you start over after a miss. That could serve to put some pressure on sinking the 8 (or 9 when you use it instead of the 8).
I would think another potentially good drill to address your issue would be to throw out 2 or 3 balls at random with the 8 or 9. Run out in order. This would have you shooting the 8 or 9 more often. Keep track of your wins/losses with a win being a complete run out. Again, puts a little pressure on hitting the 8 (or 9).
It seems obvious to me you've created a mental block with the 8 or 9 but are otherwise a very good player. You just need to get your confidence back when shooting the 8 or 9 when under some pressure. I think once you have a few league games where you drill the 8 and 9 center pocket, your problem will likely disappear rather quickly. I'd also check the grip when under these pressure situations to see if you're not tightening up when shooting these shots causing a slight pull or push.
