The speaker is writing to someone whose approval has always mattered enormously, and the poem is about the pain of never quite earning it. They have tried to please this person and avoid looking bad in their eyes, yet they feel they keep failing. From there the hurt turns inward. The speaker wonders if they are simply too sensitive, unable to handle rejection the way a "normal person" could.
The third stanza grows bitter, even angry. The speaker imagines a cruel god who made them different, and compares the world to people who look down on others of the same height over a single "uncontrolled difference." The poem never says what that difference is, which lets it feel broader and more personal at once.
The closing image of a "glass heart" and "cracked wounds" sums up the mood: fragile, tired, and lonely. The masks the speaker wears hide the damage rather than heal it, all for a little appreciation.