How can I gracefully exit my current post-doc position for a new one?
I changed from one unfinished postdoc to start a new one.  In your list of reasons, there is one that easily trumps all others:


  the new position is in the same city that my partner lives in, and we will be able to live together.


Tell your current advisor that you are moving to another postdoc so you can live with your partner.  Any remotely humanoid advisor will understand and accept that.  The reason for my move was exactly this.

All your other reasons are less important, and may be interpreted as implicit criticism of your current place.  Since you have the two body problem, you can move without mentioning any other reason.