Sometimes I get caught thinking about a past experience I've had when I'm feeling really depressed/upset. It's usually after I've suffered some kind of failure. One moment I remembered was when I was in the hospital after I had attempted suicide and I was eating lunch. Then a nurse walked by and stared at me. She came up and grabbed the utensils from my hand and was like "no..." and came back with a plastic spoon. It just made me awful as a human being and whenever I feel awful as a human being I'm reminded of that.
But other than that I don't think about the past too much. It only really exists and stored representational/informational content in our neurons (this is contentious but I don't feel like doing philosophy of mind today so it'll do).
Take care Naugabury.
*Edit* I'm with Cracked. Maybe it's not such a good idea to be stalking them. Sorry I didn't mention anything about you in my post. It was a little selfish...