This is the best thing journals and lists are for! To look back and remind yourself of where your head was at at the time.
I wrote out a list of plusses and minuses about moving here, and still have it. I would always pull it out whenever I got homesick or just depressed in general.
I haven't done a plus/minus list for going back, but every time a thought occurs to me about what I will miss and what I won't miss, I write it down.
Like when I'm stuck in-between King's X and Camden Town for 15 minutes, and not even the driver knows why!
Or, on the positive side, I take more pictures of things, and realize how lucky I am to have had this time here, and also know that most of the things I enjoy about the U.K. are almost all things I loved as a tourist, and can again!
Next time I come back I'll just know how to get around better!