The central problem expressed through the speaker's use of negative words and phrases in Petrarch's sonnet 18 is:
He cannot capture his beloved's beauty in verse.
In the first 8 lines. Petrarch is describing all the aspects that he adores about his lady and how her beauty is capturing him. Then is the next 6 lines, he reveals how love has captured him in a time when he wasn't even ready for it.
Love was a journey of ups and downs for him.