Ummm... you're surely not alone in this one. I felt this way for about 4 months after my IM. After devoting every second of 9 months to training and focusing on one thing- I had nothing left to work towards. I gained weight, became flexible (because yoga is all I did for those 4 months), and was "lost." I finally had to sign up for a spring marathon just to give me something to look forward to, and to give me a new focus.
Sometimes when we achieve our goals, we pick everything apart and are very self-critical. Having a new goal is the best thing to get you "back in the game." And you did sooooo great in your race (tell your brain to shut it and back off the re-thinking).

Are there any races coming up soon (5K, tri, bike race)- anything that will give you something to work towards??