There are so many snap decisions a player has to make while playing that it's mind-boggling that players often increase the mind-load by adding unnecessary decisions. In table tennis, there is never a decision on whether you have to move; you always move - or more specifically, you always assume you have to move. Maybe once in a hundred shots you might not have to actually move, in which case you prepare to move and simply don't. The huge bulk of the time you will move, even if it is one inch, if you want to play your best shots instead of awkwardly adjusting while reaching.
So, for now on, ban any thinking about whether you have to move. The only decision is in what direction and how far. As the opponent is hitting his shot, flex your knees slightly, perhaps even do a tiny bounce, and finish set to move in either direction as the opponent is hitting his shot. (Watch video of top players to see how they do this - don't watch the ball, focus on one player, and for this, just watch his feet.)
Remember, in table tennis, you aren't an oak tree; you are a squirrel scampering about the court!