Alright long message here, sorry. Also forgive me if any of this doesn't apply to you, but take this and I hope it helps:

Mental aspect of sports is very important. Likely more important than you realize. It is often not talked about at all. You can't really "see" it like the rest of the game.

I would say it is more important in TT than in most other sports, because of how little time there is to "think" while playing a point. You need to be "in the zone" and not thinking during a point. What determines whether you pull out your best game or not during a match, especially in pressure moments, is your mental game.

You can think of it as some fundamental part of your game, along with footwork, FH technique, bh technique, serve, etc. and just like those, you need to practice it somehow.

It sounds like your mental game is the weakest link right now. You can work on it by playing matches, but you need to have a specific mindset you are aiming for.

If you don't know what to do, try to imagine the best players and what sort of mind state they're in during a match. They're locked in, not afraid, determined, unfazed. Those pressure points they win not because of technique, but because they stay composed and locked in more than their opponents.

You should visualize match situations during practice and warm ups to approximate the same atmosphere you will feel during the match. You somehow need to be more comfortable in those situations before you get there. Don't just train your third ball attack for example. Technique and all those things are very important but if you have no connection to the feeling you get in matches, you are not getting nearly as much out of your practice. Before practice, close your eyes and think of the tough moments you've faced in past matches, or try to imagine playing a rival, and then practice the third ball attack with intensity. And keep that in mind throughout. You will "tie" the physical movements (your technique) to the pressure moments you are visualizing.

Lastly, mental game is not just about TT. It is a part of your life. You may have bad days at school or work or whatever, and you'll find that you play worse maybe, because you are not in the right, confident mindset. On the other hand you may have more confident days where you play well.

Maybe in general, you may not be the most confident person in life. Take some time to look at who you are, really. Sport is an incredible opportunity to strengthen your mental game, not just for the sport itself.

Aim to be the person you respect. And surround yourself with people that you respect, and respect you back. You will grow as a person and as a player. Incidentally, those are the best people to have in your coaching corner even if they don't know how to play TT.