He explains that a person's speech has the ability to raise up those that hear it and make them more spiritual, or to bring them down and make them more physical.
Mosheh naturally assumed the fault was with himself and that when he came a spoke to the people that he had brought them lower, made them more physical, and brought harsher judgement upon them. Therefore he questions his own purpose in HaShem's plans once again suggesting that perhaps someone else could be better suited to this particular mission.
This is a useful lesson to keep in mind, not only can our actions and speech affect us, they will have an effect on those that hear us. If the effect isn't positive, it's going to be negative. (chas v'shalom) We must work with great discipline to ensure that our words are aimed to elevate those around us.