Complicated positions

I recently played a game were my opponent played a najdorf-sicilian with black, and i ended up in a position that was simply... too complex for me. I normally prefer cristall clear play (although always aggressive and quite alot sacrifices) were my plans are clear, but in this position i wasn't sure what to do and analysis showed afterwards that I did mistake over mistake (although mostly unpunished by my opponent).

I ask myself if everybody is struggling with those closed, cramped positions or if i have to improve my play in that part of the game (i'm not even sure on how to do this...)

There are certainly very difficult positions arising in the Najdorf and your opponent playing black probably knows the line much better than you. 

The only way to master those positions is to study your loss and  go beyond running the computer analysis. A 15 10 game is pretty short to try and figure everything out over  the board.

Or you can play a closed Sicilian.

/ If you post the game here and point out the position (s) you were confounded by, others may be able to help.

i know that i got outplayed in the endgame but that was due to time trouble and concentration... but in the middlegame i did a lot of obvious mistakes, which is not so typical for me in other positions

This is a complicated position where you were easily winning. Your move e6 was a blunder. exd6 would have opened up the position for you with your opponent's king stuck in the middle.

This post is totally contradictory.

Crystal Clear play with lots of sacrifices?  The only thing in life that is crystal clear is the fact that Crystal Clear Pepsi back in the 90s was the most disgusting carbonated beverage ever invented, and there are some real disgusting ones out there, like Cheerwine and SunDrop!

That said, your question about closed, cramped positions?  The answer is you have to improve your play.  I get closed positions all the time, and often cramped, and still win!  That's pretty much my life as a King's Indian and French player!  Both result in closed positions, and both see White with a space advantage if he doesn't do anything stupid like play the Exchange or Tarrasch against the French!  Same thing goes for the King's Indian.  e4-d5 vs e5-d6 and d4-e5 vs d5-e6 both see White with a space advantage!