The vast majority of serious accidents are pilot preventable:
Stall / spin
Controlled flight into terrain
Engine failure due to a) carb icing b) fuel exhaustion
Inadvertent entry into IMC with consequent loss of control
Cherokee trying to take off, four up, full tanks, upslope, long wet grass, not qui-i-ite long enough and trees at the end...
Just tell your girlfriend that only stupid pilots make these mistakes and you're not a stupid pilot.
Or you could put it another way -- if you're going to die in an aircraft it is most unlikely to be due to mechanical engine failure and much, much more likely to be due to something you could have prevented.
I leave it to you to decide which will be most reassuring for her.
QDM