Masturbation does not lead to weakness. Its a cultural belief that masturbation leads to loss of energy and fatigue but it hasn't been proved to be so. However excessive masturbation becomes a problem when it starts hampering one's day to day activities or social functioning. It can become a compulsion when one wants to curb the habit but one isnt able to do so inspite of repeated attempts. If that is the case you should consult a psychiatrist. Fatigue and weakness can also arise due to other causes, physical or psychological. The cause has to be treated to get rid of the fatigue.