Watch any great pro player- very simple- on each individual shot---- total commitment to the shot as they pull the cue back and total focus as they stroke the cue forward- no matter the venue, the opponent, or the score.
Either your mind has the ability to not get in the way of this or your mind cannot dismiss some outside thoughts-- so it is either natural ( uncommon but a blessing if you have it) or the mind is trained by constant exposure to higher pressure situations.
For some people, all the pressure exposure in the world won't solve it- everyone's mind is wired differently - that is why some people just can't stomach competitive environments or conflict situations.