Great post Ratta!Good posting
But the main thing is: To detect if it s really just a mental thing (what i cannot believe really- jmho)
I would say it s both, like it usually for the most players. The more bulletproofed your fundamentals and mechanics are, the more you practice-the more self-belief you will have. And this will have also effect on your game *under pressure*.
To start here talking about the mental game mastery would be a shot in the dark- there is no *mental game in a nutshell*
A little help from a strong player or an instructor would help for sure.
In the example the OP describes, I would recommend a balance of developing a better physical game and letting go of the expectations. A tell tale sign that his problem stems from having greater expectations from his performance than what he truly believes he can consistantly execute is the excessive anxiety that he recieved before the competition. Excessive being the key word there.