I'm familiar with this as well. At some point you start to rush through the environments and after that you just fasttravel. Both diminish the experience but you can't help it as at some point it just becomes stale. Lately I've had a lot to do so I don't have much time for Skyrim, and when I do find time than it is much more enjoyable than when I got to play it for weeks in a row.
Of course at some point it remains stale. At that point you're so familiar with the voices, with the animations, etc that you just can't play the game anymore even if there are new quests via DLC. I don't think this can be avoided, it would happen to any game if it had enough content to keep you entertaint for hundreds of hours.