The lover lights the world for the speaker. It is interesting that you see the the speaker as "she" for there is no indication of gender, though it certainly might be a woman.
The lover brings light into all corners of the world: "Then thou, shadow shadows doth make bright,/ How would thy shadow's form form happy show/To the clear day with thy much clearer light/When to unseeing eyes thy shade shines so!" (Even the lover's shadow is brighter to the speaker than the acutal light of day.)
The final couplet, "All days are night to see till I see thee,/And nights bright days when dreams do show thee me," reinforces the theme of light into a world that might otherwise be dull and colorless. Even if the lover cannot physically see the beloved, the mere appearance in dreams can satisfy.