Voted

It's definitely your driving style. Besides that, driving with keyboard is really useless, simulations shouldn't be played with keyboard .
Just start driving with mouse and you'll be able to be much smoother with brakes and throttle, so your tires would last longer.

May I suggest adding a key combination (eg. Ctrl+Pause) which would activate another shiftercfg.ini, which would be set up in a such way that car would hold gear/shift up at higher revs in order to simulate Sport button on auto gearbox?