Whats does this poem by Emily Dickinson 'To lose thee' mean?

The first part is easy to figure out, but the second one is almost impossible. But I'm sure it may have a lot to do with the first part. She's using the Caspian Sea to compare, i believe, herself to the first part. But to help us find out the meaning, you have to know what perquisite means. Perquisite- an incidental payment, benefit, privilege, or advantage over and above regular income, salary, or wages. I think it is just coinciding with the first part, saying that to "lose thee-- sweeter than to gain" and also "the sterile perquisite" is a specific person.