It's a clever reworking of Plato's cave allegory.

The lover is presented as the Ideal of Beauty, which all earthly beauty is but an imperfect reflection of it. The previous mistresses that the speaker had a relationship with were mere fantasy(dream) of the lady that he is now in love with. It's a quite common conceit in Renaissance lyrics. However, the expression 'desired and got' is an original line of John Donne to refresh this overused cliché.

Source:

1. Nassaar S., Plato in John Donne's 'The Good Morrow' (2003)
2. Book: John Donne, The Complete English Poet (1971)
3. https://faculty.washington.edu/smcohen/320/cave.htm

Donne is envisioning a small area as equivalent to a more enormous space. It's an example of microcosm that views human nature or a well balanced natural phenomena as an perfect example for understanding a bigger world and the order of the universe.

The Good-Morrow is an aubade, a love poem sung at dawn that greets the morning by recalling the pleasant night spent with the lover and the togetherness they shared while also lamenting as they realize that they should soon be parted.

In The Good-Morrow the greeting aspect of aubade is particularly emphasized, celebrating the astonishing power of love that transcended them from individuals who dwelled on the unsophisticated pleasure to wholesome, perfectly balanced souls that are awakened in a new world.

To read other examples of John Donne's aubade, see The Sunrising.

Source: https://poets.org/glossary/aubade