I've done a lot of reading and youtubing about this over the last year as it's holding me back personally.
Mental strength is the ability to be in the present moment. This is opposed to thinking about the future / outcomes or the past / mistakes in the heat of a match. This happens because of the possession of a core belief system that has developed over a lifetime, and it takes significant effort to change into something healthy and useful.
Basing your self-worth on outcomes, being too attached to goals, and having a belief system about what 'should' happen / have happened / ought to happen / deserve to happen, is very rigid, unrealistic and leads to unstable confidence and a failure to perform in the heat of the moment. While things are working in your favour, especially at lower levels, things seem ok, but as soon as you are challenged at a higher level, you will inevitably fail and it will lead to burnout, making it much more likely that you will hate the sport, feel bad about yourself and eventually give up.
I experienced this myself a couple of years ago. It's much more serious than it sounds. Almost everybody has issues in this area without realising it. I notice their inner/outer critic dialogue between points and games - lots of 'should', 'most', 'got to' etc. The phrases that contain these words create a negative, constricting feeling, because the perceived consequences for failing are too high. They come from a feeling that if you fail, that this a reflection of you as a person, and can lead to a feeling that a current limitation is much more permanent than it actually is. I still catch myself doing this harmful self-talk too, when things don't go my way or I make a mistake. But I've done the education and can set myself on the path to improving it. This is absolutely necessary for continued growth. This is why all pro athletes work with a sports psychologist.
When this type of thought pattern occurs, you can't be in a being mode of mind - being in the present. Instead your brain is multi-tasking between thoughts of the present, past, future etc, making things really difficult for you. Your reptilian brain is firing, causing a freeze / flight response that is an overreaction for a game of TT, making it virtually impossible to play in a normal, relaxed way. You try too hard, over-hit everything, try to end the point too soon, and it becomes a trap loop what just keeps getting worse.
In other scenarios, you can play safe because you are too scared of blowing a lead / losing a game where you are ahead - and that allows your opponent to make a comeback. Another thing that can happen is not trying / competing properly due to fear of failure. If your ego is too attached to your performance (fragile sense of self worth) and possess a delusional belief that you are great, and the opponent isn't, you can become so scared of losing (and breaking that false reality) that you don't put in a full effort. Then you have an excuse for yourself as to why you lost - lack of effort, not lack of skill. A kid I played recently seems to have this deep-rooted issue. I have to admit I had this issue too when I was young, and I probably still have some elements of it that pop up from time to time.
You can see athletes with all these issues in every sport.
Instead, stable confidence comes from removing over-attachment to outcomes and goals, they take a back seat to a personal value system based on what you can control, instead of what you can't. With self-worth not caught up in the outcomes of a match or current ability. It's much more realistic and allows you to let go of mistakes much more easily, then return to the present.
This can be developed by making a list of your current beliefs and attitudes that cause that feeling of constriction during a match, then coming up with healthier, more realistic alternatives that provide a feeling of space. However the first step is mindfulness (eg: box breathing), you need to practise this for a few months to develop the ability to intercept your thoughts, recognising when your brain shifts to this mode. Then you can take note of what you are telling yourself, write it down and work on it later on. Anyway there are plenty of books out there that are helpful for this,
this one is the best i've come across.
And
here is a video which is very interesting, a real battle and incredible mental toughness at the end. Also lets not forget Michael Maze vs Hao Shuai, it's really interesting to watch the full match. The facial expressions and body language say it all.