In just a few lines, the speaker tells someone what being near them feels like. Two reactions are offered: being "blinded" or being "mesmorized" (the poem's own spelling). The "either...or" suggests the speaker can't quite pin down which it is, because the feeling is too big to name neatly. Both words point to the same thing, though: this person overwhelms them.
The last line explains why. "Because you're the sun" turns the whole poem into a single image. The sun is bright, impossible to look at directly, and the center everything else seems to orbit. It also gives warmth and life. So the comparison hints at admiration, and probably love, but with a hint of helplessness too, since staring at the sun can hurt.
The mood is quiet, awed, and a little breathless. The short lines and the dash before the final reason make it feel like a confession said softly, with the speaker almost surprised by how simple the explanation is.