Musar for Eruvin 107:3

A piece of good advice for guarding the tongue: Even if he has already become habituated to it [(i.e., to a certain kind of lashon hara)], G-d forbid, and his yetzer has overcome him in this area, so that he himself does not "feel" the lashon hara that issues from his mouth, still he should not despair, but he should rest assured that by reviewing the din several times, a transformation will take place and the thing [i.e., the din] will become fixed in his nature, and, in the end he will "feel" [i.e., he will be sensitive to] every [article of] speech that leaves his mouth; and even if it be only the "dust" of lashon hara, he will feel it with his tongue at that time. For the Holy One Blessed be He has implanted in a man the faculty of "feeling" [i.e., sensitivity]. And He has done it in such a way that he feels it more when the "thing" comes in contact with his tongue. (This is similar to what Chazal have said (Chullin 17b): "The master examined it [(the slaughtering knife for kashruth)] with his mouth.,") And this faculty is of great avail in the learning of Torah, so that when one brings the thing [(what he is learning)] from his mouth to his tongue, its correctness is "tested." As Chazal have said (Eruvin 54a) on Mishlei 4:22: "For they [(words of Torah)] are life lemotzeihem"… to those who utter them ['lemotzi'eihem'], with their mouths." And the same [phenomenon] holds true for all aspects of speech; but it is only because of his great habituation to superfluous speech and [his tendency] to pay no heed to it that this faculty of his has become greatly weakened. But if he follows our advice (And he certainly will be attentive to what he says from that time on), this faculty of feeling will gradually strengthen itself in him, little by little until it reaches ifs full development.