A systematic trader should (and after a while always does) know that things don't work forever and that you can get carried out on an event etc. With regard to uncertainty she's similar to a discretionary trader; in fact I'd argue that systematic trading is a continuum with different traders falling on different spots on the spectrum. Decisions about scaling risk up/down, changing parameters or shutting down specific alphas can all be very emotional decisions.Getting into systematic trading is a sure way to cure this.
It's not about the statistical approach (every seasoned trader understands that he's essentially tossing a biased coin), it's about the emotional acceptance of the uncertain outcome. If you are afraid of losing money, meditate on it like the samurais mediated on dying (see here: http://strivingforfreedom.com/death-meditation/). If that does not help, hire a psychologist. I have, unfortunately, seen very smart systematic traders who could not deal with the mental aspect and have, at times, struggled with it myself (it is especially hard after a long flat period or a drawdown).

