We cannot know his legendary head with eyes like ripening fruit (1-2)
There's certainly something awe-inspiring about the fact that this statue can never be fully "known"—even its state of decay (a missing head seems pretty significant) doesn't diminish its power. We get the feeling that if it did still have its "legendary" head (or in German, unerhörtes, which means unheard of, outrageous, inconceivable), we might not be able to handle it.
Otherwise the curved breast could not dazzle you so (5-6)
Okay, we could have picked any of the lines in the middle section of this poem to express the speaker's utter awestruck-ness (if that's not a word, we're making it one now) at viewing the statue. The inner "brilliance" (3) that he perceives is a result of his own amazement at the beauty of the object, and it "dazzles" him like the sun.
from all the borders of itself, burst like a star (12-13)
Again, the image of dazzling light comes up. The statue, in the speaker's view, cannot contain its own radiance, the piercing rays of pure beauty that are so powerful in this image that they have a certain violence.
for here there is no place that does not see you. (13-14)
The sense of awe the speaker feels is also tinged with a kind of fear. This is a pretty common occurrence with descriptions of awe—thus the earlier common usage of the word "awful" to describe something that is terrifying and beautiful at the same time. The notion that there's a higher power in the world—whether it's beauty or divinity, or both, or neither—is both great and terrible.
You must change your life. (14)
This is the real kicker in this poem. The speaker is so overawed with this encounter with the aesthetic realm that he feels—as we are meant to feel—like his life has been directly affected.