What is Shakespeare talking about in his Sonnet number 87?

Goodbye; you’re too valuable for me to hold onto, and you probably know exactly what you’re worth. Your high value gives you the right to leave me; you have severed the ties that bind me to you. For what hold do I have over you except the hold that you choose to give me, and how do I deserve such a treasure? There’s nothing in me to justify such a beautiful gift, so my right to possess you is reverting back to you. When you gave yourself to me, you didn’t know your own worth, or else you were mistaken about me, the person you gave yourself to. So the great gift you gave me, being based on a false estimate, goes back to you now that you’re able to make a better judgment. Thus, the time in which I had you was like a flattering dream; while I was asleep, I thought I was a king, but when I woke up, I found that was not the case.