This stuff goes through my head too. The first time I read that the sun would die in however many billions of years (just a couple months ago) I panicked. It's a tough one. Sometimes when I think about it I feel infinitely calmed, just thinking that I am such a tiny part of the whole and so really, I am not the center of the universe and the pressure is off of me to do anything other than sit back and enjoy the ride. However, since I'm too uptight to do that (haha) I usually just end up obsessing. When I'm dissociating it really freaks me out and all I can do is force myself to think about something else.