Let's get the nitty gritty stuff out of the way, shall we? The odd-numbered lines of "To Lucasta" are in a meter called iambic tetrameter. All that means is that there are four (tetra-) iambs all in a row: daDUM daDUM daDUM daDUM. So, for example, line 9 goes a little something like this:
Yet this inconstancy is such.
The even-numbered lines, on the other hand, are in a meter called iambic trimeter, a line that consists of three (tri-) iambs. Line 8 is a good example:
A sword, a horse, a shield.
A word to the wise, dear Shmoopers: whenever you see alternating lines of iambic tetrameter and trimeter, with an ABAB rhyme scheme to boot, you're reading ballad meter. And now that you know what it is, we promise you'll see it everywhere.
As is often the case, Lovelace can't leave good enough alone. He wants to jazz up his meter by replacing some of his iambs with other types of feet. Just look at line 5:
True, a new mistress now I chase,
That first foot goes DAdum, instead of daDUM. That makes it a trochee, ladies and gentlemen—every poet's favorite metrical substitution (we bet you didn't know poets even had favorite metrical substitutions).
When poets like Lovelace mess with meter, you can be they're doing it on purpose, and not just to keep us on our toes. In this case (and in many cases), a metrical shakeup can add emphasis to a word, image, or idea in a poem. In this case, it highlights the word "True," which here means something like "granted." So the speaker's conceding a bit of his case, as if he's saying "okay, okay, so I do have a new mistress, but…"