Why do i "forget" my hand movements when i cum?

Just when im about to cum, i lose focus on masturbating, it gets sensitive even before i cum.So instead of shaking my dick super fast, i just do it slowly, and let it out by itself. And that prevents the semen from exploding, intead it just flies away a little bit.I just cant shake is fast when i cum at the same time, what should i do?....