Faced with the overwhelming prospect of leading a nation, Moses feels utterly inadequate due to his severe communication limitations. Having exhausted his previous arguments, he turns to God in desperation and begs for mercy, feeling fundamentally unsuited for the prophetic mission [אור החיים, אבן עזרא]. This profound plea can be understood as a willingness to accept divine punishment rather than take on the role [אבן עזרא בשם ר' יהודה הלוי], or as a humble admission that the flaw guaranteeing the mission's failure lies entirely within himself [אור החיים, שפתי כהן].
The primary approach among commentators is that Moses lacked eloquence, feeling he had neither the talent to persuade the masses nor the confidence to stand before kings [אבן עזרא, ספורנו, מלבי״ם, קאסוטו]. Others suggest his difficulty was practical rather than rhetorical. Having fled Egypt in his youth, he had forgotten the language and knew he would struggle to converse in it [רשב״ם, חזקוני]. Alternatively, his character was fundamentally that of a man of action, as demonstrated when he saved Jethro's daughters and struck the Egyptian, rather than a man of words and negotiation [קונטרס חיבה יתירה].
Moses points out that this struggle is not new, having carried this burden since his youth in Pharaoh's palace and throughout his time in Midian [ספורנו, העמק דבר]. He had secretly hoped that the sheer experience of divine revelation would miraculously heal his speech. When he realized that his impediment remained even after God spoke to Him, he concluded he was simply the wrong man for the job [אור החיים, צרור המור, אברבנאל]. Some commentators note that the prolonged nature of his struggle mirrors the dialogue at the burning bush itself, which unfolded not over a single day, but over seven consecutive days of God urging him to accept the mission [רמב״ן, רש״י, טור הארוך, רבינו בחיי].
When examining the specific heaviness of his mouth and tongue, the primary approach among commentators views it as a physical difficulty in articulation, specifically a struggle to pronounce certain phonetic groups involving the teeth, tongue, and lips [רבינו חננאל, הכתב והקבלה, תולדות יצחק, אברבנאל]. This physical defect is traced back to a childhood incident in Pharaoh's palace, where an angel guided the infant Moses to place a glowing coal in his mouth to save his life, resulting in permanent burns [רבינו בחיי, שפתי כהן]. Conversely, a different perspective completely rejects a physical defect, suggesting instead that his heavy speech was the result of prolonged isolation and deep immersion in spiritual matters, which detached him from normal human conversation [רלב״ג, חומש קה״ת]. Taking this further, some explain that Moses's very essence was pure Torah, elevating his communication far beyond the confines of ordinary human language [צפנת פענח].
The lingering question is why God did not simply cure His chosen messenger. One explanation is that Moses, whether out of a deep reluctance to accept the position or out of profound humility to avoid overshadowing his older brother Aaron, never actually prayed to be healed. Consequently, God did not cure him but rather promised to guide his words when the time came [רמב״ן, רש״י, טור הארוך, רבינו בחיי]. A deeper approach suggests that God intentionally preserved this impediment through divine providence. By choosing a leader who lacked charisma and brilliant rhetoric, God ensured that when the Israelites were redeemed and received the Torah, it would be undeniably clear to the world that their salvation was achieved through divine power and miracles rather than human persuasion [אברבנאל, פרדס יוסף].