I've struggled with this too and here's what I've learned.
It all starts with confidence in your strokes. To be confident you need to trust that a relaxed, full stroke will achieve your intended shot result.
How do you learn to trust your strokes?
1. Hit lots of the same stroke over and over observing how your body FEELS in a relaxed state relative to the shot result. This is the muscle memory you're trying to achieve. Use a ball machine or, better yet, a hitting partner who can hit consistently. The hitting partner is better since he'll naturally throw in some realistic variety in shots.
2. Don't mess around with your racquet and strings. Find a set up that feels right for you and STICK WITH IT. Changing frames and strings and tensions constantly ruins your trust because each change will cause different shots given the same stroke. So you never develop a relationship between stroke feel and shot result.
3. Watch some higher level live play. The first thing you'll notice is the fearless way higher level players "strike the ball". We rec players often poke the ball or hit the ball. It may sound like semantics but it's an important distinction. Focusing intensely on the ball and striking it fearlessly in a specific manner with the explicit intention of achieving a given result is very different that merely hitting the ball with the abstract hope it will get somewhere over the net.
4. Visualize yourself as playing as a higher level player confident you can do it. When I'm playing confidently relaxed I sometimes visualize myself as a favorite player. I feel like my footwork is better and my strokes worthy of hitting winners. I become those other people emotionally in order to unleash my confidence and trust my strokes. We've all hit amazing shots at various points. The challenge is doing that consistently. Visualization can support that objective as it builds confidence.
Finally, as Obi Wan said, "Let go your conscience self and rely on instinct". If you play selfconciously, worried about what others will think, then you'll play stiff and play worse! Playing with instinct frees you from this social cage to play your best. It may be you don't play all that stiff and only did so aware that you were taping yourself for review on TT!
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