At first, the speaker is annoyed by sunlight pushing through the blinds and fights against it. Then the poem turns: the sun becomes a person, someone the speaker calls "my sunshine." This person is "sometimes harsh sometimes warm," always pretty, and sometimes absent. The speaker isn't sure who or what this is, and the poem never says. It could be a lover, a friend, or even a hopeful feeling.
The middle of the poem is the most vulnerable part. The speaker wonders where this person goes when they vanish and whether they'll come back, and then admits, "because I need you." That plain honesty carries the poem's feeling of longing and uncertainty.
The ending shifts to resolve. The blinds, which first kept the light out, now stand for the speaker's resistance. This time they choose to open up and let the light "fix what's within." The mood is quiet, tender, and hopeful, with a sense that the speaker is ready to accept help.