Simple answer ( imo) :
2 main differences between training and competitive matches is this:
1. Higher level of stress in mind and body during matches compared to training
2. Different thought processes, goals and values during the match
Stress is a complex phenomenon but this should help you get an idea of it.
Simple steps towards solution:
Regarding stress:
- increase intensity and competitive mindset during training ( training with higher stress)
- lower stress using techniques, phrases, reminder or things to calm your nerves during the game and bring you into the moment and a calmer state of mind ( playing with lower stress )
( Visualization, Cues and reminders about your goals, Conscious deep breathing, some movement shaking it off. etc. ...)
- In the long term,
figure out the root cause of what is stressing you, and learn to deal with that
The fear of losing is just the most prominent in the moment, but its not what you actually fear
you wont die or feel a massive pain by losing the match, you wont become homeless and your loved ones will not leave and hate you, you are not in an immediate danger or threat during a match ( i hope so at least)
find out what is on the deep side of it and deal with it,
In the end it will be something that, on a subconscious level, threatens your inner value, and your body goes into alarm mode to fight that danger.
1 wide spread issue is FOPO ( fear of other peoples opinion)
In short words:
You value other peoples opinion and their expectation of you, and if they dont see you as "valuable", or YOU think they think like that
-> your body goes in alarm mode to fight that -> you play bad and get into a downward spiral of playing worse and worse
Once you find the root cause, figure out your goals and values that will maek
It is a difficult and long process to really deal with that, it appears throughout your whole life and takes some effort to get a hang of it.
2. You can think differently during training and in the match.
1. For a movement or technique to really stick with you so much that you can use it in a match takes time, effort and deliberate practice, once you can train the movement ( for example forehand on underspin) with the intention to play the ball in a certain way and not to do the movement itself ( like multiball ) and then do it consistently at least 9/10 times, you get confidence, ability and a mindset which enables your match gameplay to improve and match your training
Also practice intensely and deliberately, make hitting a shot more important than it actually is, make your mind and body get closer to a competitive state
2. In a match, Dont try to win, try to play well, point by point, moment by moment
set simple, achievable goals that you feel confident in achieving
Try to learn something from every match,
dont identify with how you perform
TLDR:
Lower match stress,
increase training intensity
Find the root cause of whats stressing you out, set your (life and TT) goals and values accordingly
Dont try to "win" in a match, play point by point and learn
Most importantly, dont lose hope, keep believing and practicing.
Experience will always be your best teacher.