There are several synonyms for the phrase "lessen the self-confidence of". One of the most common synonyms is "undermine", which means to weaken or erode someone's confidence in themselves or their abilities. Another synonym is "diminish", which refers to reducing the level of self-assurance and conviction a person has. "Disparage" is another term which means to belittle or criticize someone in a way that can impact their self-belief. "Debase" is used to suggest lowering the level of self-esteem or dignity someone possesses. Other synonyms include "devalue", "demean", "deprive," and "reduce." All of these terms refer to actions that can impact a person's confidence and self-worth in various ways.