A big question is how does one keep their mental composure when playing chess... or perhaps doing anything in life.
Sure, when you're on a roll you're on a roll. What happens when things get complicated? Your whole focus tends to break down. You make a move and seconds later you think... "gah I can't believe I overlooked that important detail". Then it goes to... "I hope they don't see that mistake". Then it goes "how do I make up for that blunder".
Concentrating can make all the difference. Even if you blunder horribly, if you concentrate hard you can still win the war on attrition by keeping your opponent on edge and making up lost ground.
So what is the best way to concentrate? Well... concentration often starts before the match. Practicing good structure to your thinking goes a long way. Good structure to thoughts are applied to each move not just to some. Just because you have blundered or the situation looks completely foreign, it doesn't mean sticking to your thought structure will prove any less valid.
Armatures tend to break away from their practice is scary situations. Sometimes it's safer play, sometimes riskier gambles. Regardless of the direction, the intent is always negligible and tends to domino uncontrollably. That means you giving the keys to your opponent.
So with all things said and done practicing structure to your ideas is key. The practicing part engraves this into your system so your knee-jerk reaction is using your brain and not just your nerves. If you find yourself not concentrating... slowdown. Forget the clock... for a moment, and add a bit of structure. It will save you time in following up and will, if nothing else, give you direction.