Musar zu Schemuel I 17:36

Another way of explaining our Psalm may be that David felt the need to apologize for taking credit for his humility, i.e. he apologized שלא גבה לבו, for having his humility become common knowledge. This in itself would rate as a form of boastfulness. He apologized, for not having remained silent, i.e. אם לא שויתי ודממתי נפשי. Whichever may be the case, the subject of this Psalm is the extreme importance of practicing humility.