Personally, I was in a serious slump beginning this season. For me, the key to improvement was to relax. By this I mean relax in my preshot thoughts, relax my hand in my release, relax through breathing in a controlled way at the line. The biggest of those for me has to be the release. When I struggle, I tend to get too stiff in the fingers and hand in general. That leads to a lot of over and under, and poor directional control in my game. A softer hand, not limp, but softer than a rigid hand helps me to get the ball down the lane and gain control of the reaction. I have no idea if this will apply to you, but it can never hurt.
Of course the other thing I have done is get myself a Blueberry Buzzsaw. That helped more than anything because I've been between 625 and 685 in my last nine sets.
Give yourself a break, don't beat yourself up, especially when you're trying to perform. Good Luck
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Ken Stoner