Concentration

Well first off,  the reason you play better when you are in a more serious mood is because you are concentrating more.
Self control can help a lot in concentration.
People don't really understand what concentration is ---  and think that because they decide to be focussed they are concentrating,  which is false.
Concentration comes form looking at some thing and just by reading the information that come to you, so it simple.
If you want to play well or gain a better understanding of the game just stop and look the position.
First off look at where all your pieces can move.
Don't make a move.  Dont try to find the best move. 
Just look at all the places where your pieces can move to.
Just by doing that you gain a bigger picture or understanding of your game.
Then use the basic rule to guide your move.  Again don't look for the perfect move.  Just build up with basic guide rules given you earlier.   Develope your pieces and control as many square as you can ( focus mostly on center squares).
And remenber chess is not about finding the best move because if you don't understand why you moved a piece  then it a useless move... so just use all your chess knowledge and make the move that you think is right and you will learn to make deeper and deeper plans. And then when you get proper understanding of what is going on then look for the best move.
Dont get fustrated ---  just try to understand what you think is a wrong move and why you made that move.
Just doing that you might not see everything but you will learn to analyze  a game and get better understanding of what makes a good move 'good' and bad move 'bad'.