In table tennis and other sports, your subconscious is trained in a training environment, where there's no pressure and so players can relax and just drill their shots or play practice points. With no pressure, there's little hesitation or rushing of shots. If one of those happens, the player or coach makes sure to fix it, and without pressure, it's not hard to fix.
Then you get into a match where there is pressure. A confident player does the same under pressure as he does without. But even the least, tiny bit of hesitation can completely destroy your timing for an entire match. It usually starts with a hesitant shot that misses because you (rightfully!) didn't train to do hesitant shots and so have little control over them. Trying to compensate, you then rush a shot. Suddenly your subconscious doesn't know what to do - all that training is gone as it can't time hesitant or rushed shots.
Ironically, the player often thinks he just needs to practice the shot more until he can "do it in his sleep." While that's helpful, it doesn't really fix the problem here. The real problem is if you aren't confident, then neither is your subconscious, which is what times and executes your shots.
So the real problem here is confidence. When walking, you have complete confidence you will make that next step without tripping - but you don't actually think about it, you just keep walking flawlessly. Similarly, you need to have complete confidence in your shots without actually thinking about it. The key here is sports psychology. Here are a few links. Go to it - and don't hesitate!