For my first twenty years, since yesterday, (line 1)
One of the main themes of the poem is how the speaker's sense of reality is twisted around by the calamity that has befallen him: a day without his lover. But he has not gone crazy, because he can still compare his sense of time with the "real," external time.
Tears drown'd one hundred (line 5)
Although the poem is technically about love, the speaker's ego is always at the center. He is a larger-than-life figure whose tears can "drown" a century. What he means, of course, is that he has spent an entire century crying. Also, reality gets more and more bent out of shape throughout the poem, as time expands.
Or not divide (line 7)
Our best guess is that "or not divide" means, "nor could I divide." Division is a pun because it is both a spiritual activity (division of the self) and a mathematical procedure, a kind of "computation".