"If you could see yourself just for a day" implies that our self-perception can often be different from how others perceive us. It suggests that we may be overly critical of ourselves.
"You'd see how everyone else sees you" suggests that the perspective of others often emphasizes the positive and beautiful aspects of an individual.
"And my god you are fucking beautiful" is a declaration of the speaker's awe at the beauty they believe the person possesses, emphasizing the idea that we often underestimate our own attractiveness.
In essence, this quote encourages self-love and self-acceptance, suggesting that we are often more beautiful in the eyes of others than we realize. It encourages individuals to appreciate their unique beauty and value.