IN ORDER TO SLOW DOWN YOUR OPPONENT, YOU MUST FORCE HIM TO MAKE A DECISION.
Let me explain...
During a match, most of the mental toughness comes between points. Being mentally tough means...
1. Evaluating the previous point for what you did right
2. Evaluating the previous point for what you did wrong
3. Encouraging yourself
4. Planning some general tactics if you are receiving
5. Planning some specific tactics if you are serving
6. Reminding yourself of your strengths and planning how to implement your strengths
7. Reminding yourself of your opponent's weak points and planning how to exploit them
Like I said... Most of the mental game used is between points. During the point, it is best to just "let it happen." Table tennis is so fast that you really do not have time to think much during the point, especially about your technique. During a training session, your coach might be continually yelling at you to start your backswing in a different location, using your wrist, change your racket angle, spin the ball, using your waist, complete the stroke, and hundreds of other aspects of your stroke. However, in matches, there is absolutely no time for this thought process. You need to just let it happen - let it happen as you have practiced.
So how can you use this against your opponent?
Your opponent is just smoothly ripping forehands and backhands through you like knife through butter. You have absolutely no chance because he is playing out-of-his-mind and swinging freely. So how to you make him "freeze up?" You make him freeze up by making him decide. Force him a make a decision.
HOW DO YOU FORCE HIM TO MAKE A DECISION?
The best way to force him to make a decision is to play to the transition point. By playing to the place where he must choose whether to play a backhand or forehand, he will need to decide, which will slow him down. Usually at the transition point, players will be jammed and play the ball later, which gives you more time to react.
Another way to force him to make a decision is to serve half-long. The half-long or in-between serve is defined as the first bounce (obviously) hits your side, the second bounce hits just before the half-way point on the opponent's side, then the final bounce lands close to the end line. With this serve, you are forcing your offensive opponent to make a decision to use forehand flip, loop, long push, short push, or backhand banana flip. On the half-long serve, there are many options, but your opponent might feel "stuck" when trying to make the best choice.
There are dozens of other ways to make him decide as well - vary the spin on your serve, vary the speed of your flip, vary the height of your loop, vary the depth of your chop, vary your shot selection, and many many others. The key point that you must remember is that you are trying to make your opponent indecisive. Instead of merely trying to play amazing yourself, you are trying to force your opponent to play poorly. Apply this principle to every match and your giants won't seem so tall.