We can help to better understand YOUR main strengths & weaknesses and guide YOUR development.
Start on your own to evaluate the ability to consistently hit the sweet spot, performing stable and consistent swings.
It's easy to understand that off centred shots can make the racquet twist, so to lose control.
The outcome of the shots is related to what we call the "ESSENCE OF THE TECHNIQUE"
so what it happens at the contact between the ball and the strings.
There are 4 elements to take into consideration for this key moment, together with the timing:
a. The inclination of the racquet head
b. The direction of the swing
c. The speed of the swing 
and last but absolutely not least
d. Your ball feel
 
Any other aspect (mental, physical, tactics and technical) is useful only as far it can help to optimise the striking moment; what it happens after it only serves to get properly ready for the following shot.