I got an A from Game Theory. But after that i forgot almost all the formulas and theorems. What should i do to prevent that? Do you forget math like i do?
I would take heart and persevere. Something I have learned over the years is this:
Although it may feel like the material you learned is all gone, it is actually hidden in your brain somewhere. You will find that if you go back to the books and try to re-learn Game Theory it will be much easier than it was the first time - you will keep feeling that "oh yeah..." as the light bulbs go off and everything starts to return.
It feels like I have learned some things a hundred times over, but you just have to accept that. Not all of us, unfortunately, have memories like Euler did :(
And yes, as the accepted answer to PandaBear's link in the comments says, teaching is by far the best way to remember things. Take time to slowly and thoroughly explain something to a student and you will find that it won't easily go away.