Folding KK is tough for almost everyone. It's difficult to understand what situation you are asking about, river or preflop so I'll proceed more general.
Preflop situations folding KK, in satellite situations where you are not the short stack and need not risk chips near the bubble, this goes for AA also. Maybe when facing raise or reraise from very very tight players, this is one of the more difficult spots. As you progress in the game and get better at understanding players and reading
hands this will be something you can do "easier".
Post flop is much more complicated and gets harder as you progress to river play. My best advice here is to continue to study hand reading and strategic play. Understand how your opponent's think and how your image is broadcast.
Good luck and keep learning.