The phrase you're looking for reminds me of a powerful moment in 'Naruto' when Rock Lee struggles with self-doubt. While I don't recall an exact English equivalent, the sentiment echoes lines like "Those who don't believe in themselves will never know true strength" from various shonen anime. The concept appears frequently in motivational contexts, particularly in sports manga where characters overcome mental barriers.
Interestingly, this philosophy contrasts with Western self-help approaches that often emphasize external validation first. In Japanese storytelling, the internal belief typically precedes achievement. You might find similar phrasing in 'My Hero Academia' with All Might's "You too can become a hero" ideology, which carries that same foundational message about self-worth being the starting point for growth.