Another (II)
hartless = without hart
A hart is a male deer. She's punning: the female deer is 'heartless' (anxious) because she lacks her 'hart-less' (without her male). Bradstreet loves this kind of wordplay.
dearer dear, dearer heart
She's one-upping her own metaphor—her husband is even more precious than the deer is to the hind. The stacked comparatives show she's trying to measure the immeasurable.
turtle = turtledove
Turtledoves mate for life and were proverbial symbols of faithful love in Renaissance literature. Not the reptile.
mullet folklore
Renaissance natural history claimed mullets would beach themselves to die near their captured mates. Likely false, but the myth appears in emblem books about marital devotion.
both kept by force
This reveals the separation isn't voluntary—he's traveling on business or civic duty, she's home-bound by gender restrictions. The constraint is social, not just geographical.
substance vs. dreams
Puritan theological language: the material world without spiritual (or marital) union is mere shadow. She's applying metaphysics to marriage.