The higher your combo gets, the higher the pressure gets about not breaking your combo, it's a natural thing to humans. Once you get used to high combos, you'll eventually push past it. I remember when I used to think "omg I'm on 400 combo!" whereas now I don't even take notice of my combo until it's near about 800.
As for concentration, I'm not really sure on how to improve this. I'm able to think about other things while playing IIDX, it's as if it just goes from my eyes to my fingers...
