Why is it that Yosef, the master of self-control, suddenly loses his composure and emotional discipline?
Yosef's courageous defiance of Potifar's wife's advances renders him the paradigm of self-control and restraint. "Yossef HaTzaddik," as he is known, demonstrated how far will-power and spiritual consciousness can extend, to the point where one can overcome the strongest challenges, so long as he possesses sufficient resolve and determination.
Interestingly, however, specifically his inability to exercise self-restraint seems to have contributed to his decision to finally reveal his identity to his brothers. The verse states that just prior to his unveiling of his disguise, Yosef was unable "le-hit'apek." Rashbam explains this phrase to mean that whereas heretofore Yosef had the wherewithal to overcome his strong, fraternal emotions towards his brothers, at this point he simply broke down. He could no longer contain himself, and felt compelled to truly become their brother once again. (Other commentaries - most notably, Rashi - interpret the verse differently.)
Why is it that Yosef, the master of self-control, suddenly loses his composure and emotional discipline?
The answer seems to be that Yosef's emotions directed themselves in accordance with his value system. Fraternal love and affection earned a respectable place in his hierarchy of values; his physical attraction towards a gentile, married woman was clearly suppressed by his demanding moral code. Yosef HaTzaddik mastered the art of self-control; he channeled his emotions in the directions determined by his strict ethical standards. Regarding the virtuous emotion of brotherly love, his feelings overflowed beyond control. When it came to the sinful desires of a forbidden relationship, his feelings were kept in check.
Essentially, herein lies the secret of overcoming one's improper tendencies. The stronger the resolve, the more one reinforces within him the conviction to do what's right, and the more his temptation for sin will give way to his inner will to do good. Yosef's self-control could not help him overcome his affection towards his brothers; but it was his self-control that directed his emotion away from Potifar's wife and towards his righteous brothers.