I realized that I tend to play really bad returns or do mistakes when I get a high and/or slow ball from the opponent in the middle of a tight rally. I think it has to do with the fact that all of a sudden I get time to think on what I want to do with my hit, instead of playing by intuition and reflexes. That almost always ends in messing up my shot and giving away an easy point.
How can I improve my game with on that aspect?
You are probably okay on the fast rallies because you get a lot of practice on them - it sounds like the high/slow returns are unfamiliar to you, or the sudden application of back spin is a contrast to the likely use of heavy top spin on a fast low rally.
Simplest advice is to practice with a friend - get them to send you many high shots, especially with varied spins, so you can get used to the effects of backspin and side spin.
Practice, practice, practice
My simple answer: just do it.
As a golfer, I get caught up with mechanics. After being discouraged for a period of time, I made the decision of spending minimal time contemplating before I hit my shot. As a result, I'm less focused on how I'm doing it and more focused on just doing it.
Point being, no matter how the table-tennis ball is being served/returned to you, the main goal is to return it back. In my own experience, when I would get the high/slow ball served/returned to me, I think of a spike only to hit it into the net or a fast return only to over/under hit it. Just get it over the net, and with the more confidence you get returning the high/slow ball, the more aggressive you can get.
Don't change your gameplan because things seem to be in your favor. It could change any second.
If a slow or high ball comes straight to your forehand hit it with as much forward spin as possible and it does not have to be that hard it all matters on the spin... After you can do that you can try putting a little bit of side spin on the ball as well but that should come naturaly. when you are playing these shots make sure you hit it right at them and not to there right side. Then your opponent is likely going to hit the ball to far (if he hits it at all) because of the forward spin gripping on his paddle.also the forward spin will pull the ball down toward the table because of some funny aerodynamics.
hope this was helpfull. HAVE FUN! :-)
