How to absorb the information?

I did a lot of research lately. 

I studied the Ruy Lopez version when black plays Na5, and a lot others

Lately I realized, that I can not absorb all of the informations, strategies, tactics in my game. I always forget something, I don't do very well on the board lately. 
I don't know why do I have really awkward decisions. I did my lessons, I'm at my highest at the tactics here on Chess.com, and I still do very bad on the boards. I don't blunder, but I make Innacuracies and Mistakes too often. 

When I analyse my games, I see a lot of bad judgement and I see more clear what should've I done, but when I play, I'm like blindfolded. Theoretically I know a lot more than I can use on the board. This is really-really frustrating, I lack confidence because of it. 
My only reasonable answer to this question is lack of concentration. A short pause is what I need, or the problem is bigger?