If you hit bad shots under pressure, one major cause is that you fail to complete your backswing.
Unless we have trained ourselves well, pressure makes us change what we do.
We get too careful under pressure.
That cuts the backswing short, and that's no way to hit the good shot you need.
Completing your backswing means that everything has moved back to its natural stopping point.
By everything, I mean your hips, your torso, your arms, and your hands have all turned away from the ball as far as good swing mechanics and your flexibility call for.
How far is that? The ideal position for your leading shoulder (left shoulder if you're a right-handed golfer) is right under your chin.
Getting to that position means your torso has turned about ninety degrees.
Your hips should turn half that, or maybe a little more.
Turning your hips less than that, given the same amount of movement in your torso, could lead to lower back problems.
The amount your arms turn is governed by your leading arm.
Stand upright, put that arm straight out in front of you at shoulder height, and swing it across your body.
Where its stops without being forced further is where it should stop in the backswing.
Your hands turn away from the ball when the wrists set themselves.
Be aware that there are two kinds of set.
One is the cocking of the wrists, expressed by them bending sideways and setting up the right angle that you see the clubshaft make with the leading arm at the top of the backswing, when viewed from face on.
The second kind of set is the bending backwards of the trailing wrist.
It bends only as much as it needs to so the angle in the leading wrist remains the same as it was at address.
If any of these positions get cut short, you are taking your swing out of its normal alignment and introducing new angles that will make it difficult, if not impossible, to get the clubface back to the ball square, on line, and with the proper trajectory.
The resulting shot will be the one you were dreading.
Maintaining your normal backswing begins with your mind and ends with your mind.
There is no reliable physical guarantee.
The solution is to be able to concentrate on what you have to do, rather than how you are going to do it or what the result might be.
Calm your mind and take practice swings until your mind is back on the feeling of your perfect swing.
Fill your mind with its easy rhythm and graceful flow.
When there is nothing in your mind but that feeling, step up to the ball and swing, don't wait over the ball, copying the last sound practice swing that you made.
I guarantee it will all be there, and you'll hit the shot you need to.