How often would I say, at that time, full of awe, "For certain she was born up there in Heaven!" and her divine behavior, her face and words and her sweet smile so filled me with forgetfulness and divided me from the true image that I would sigh and say: "Just how and when did I come here?" thinking I was in Heaven, not where I was; and since then I have loved this bank of grass and find peace nowhere else.
Petrarch has retreated completely into his memories of Laura and of how she made him feel. Basically, she makes him a bit stupid when she's in his presence. Again, it seems like a cliché, but remember that this is the fourteenth century—we're seeing the genesis of all these nifty ways of describing the feeling of being in a doomed relationship.
The level of hyperbole here is quite high: Laura's not only giving us a toothache—she's actually a creature of Heaven. As such, she brings paradise wherever she goes, fooling the smitten Petrarch into thinking that he's actually in heaven.
This becomes a major problem for him. Laura's divinity "divide[s him]/ from the true image" and makes it impossible for him to accept that he's living and breathing in the real, earthly world. Why is this a problem, you ask? We admit it sounds like a pretty sweet deal.
Well, he can't live in heaven all the time. Petrarch himself tells us the problem in lines 64-65: "and since then I have loved/ this bank of grass and find peace nowhere else." He can't be comforted by other human beings, only by the memories recalled by the river. Basically, his "lovey" is a spot of landscape far away from other people. The solitude and tranquility of this place, and the fact that it's a grassy plot, echo the image of Petrarch's imagined grave in stanza 2—not good.
Stanza 5 also seems to be the place where the verse form really falls into place. Petrarch's working those hendecasyllables and settenari to the best of his ability, including that gorgeous last line which clocks in at exactly 11 syllables and four main stresses: "quest'erba sí ch'altrove non ó pace." Why the insistence on regularity? What in the Sam Hill is a hendecasyllable? Check out "Form and Meter" for more.