Mental stress has its roots in the sufferer's mind.
Mental stressers' brains work very fast, but in the wrong direction. They're very caught up in the notion of perfection: they see the world very critically, pick up on the smallest details, and judge themselves even more severely than others. Their thoughts become stuck in an automatic negative mode and they pile the pressure on themselves.
A lack of self-confidence also leads mental stressers to seek other people's approval all the time. They think they're not up to scratch and dwell on their failures rather than their successes. They imagine the worst, raise the stakes and end up stressing out of all proportion. If you tell yourself you'll never achieve what you want to achieve, your reactions are slower and you generate more stress for yourself. It's a vicious circle.