Is it better for someone who adheres to the apparent Sunnah and strives to improve his character, but sometimes behaves badly, to abandon the apparent Sunnah or to continue striving?

Letting the beard grow and outwardly manifesting the Sunnah (definition) while possessing bad manners leaves a negative impression and may deter people from good. However, the remedy for this is to strive against one's self and improve one's character, not by trimming the beard. Seek help from Allah and frequently make the supplication: "O Allah, You are the King, there is no god but You... Guide me to the best of characters, for none guides to the best of them but You, and turn away from me the evil of them, for none turns away the evil of them but You."