You sound like me.
I celebrated my "almost 20lbs" last week by going out to one of my family's favorite resturants. I did everything right, sort of. I got a buffalo burger (very lean) and forgot to say no cheese, but I didn't eat but a few bites of the bun and maybe one or two french fries and a salad.
Then each day I fell off the wagon a little bit more. My face started clearing up (I've had horrible acne or hives or something all over my face since about 2 weeks into medifast) and I realized I only had about 4 days left of food. Combine all those and not only did I fall off the wagon, I lept.
BUT! I got my order in, and the next day (yesterday) I started off fresh, brand new day, all that jazz.
It's been said a lot, but it's really something you need to listen to. You can't change what happened yesterday or last week, you can only change today and the future. Make a deal with yourself that you can slip off the wagon at the next 20 pounds if that's what it takes to get you over not wanting to restart and survive the 3 day hump. You can always change your mind later and stay on it until the next 40 instead.
It's pretty amazing though. While I was off the wagon I decided to finish my stash of celery. I ate the whole bag. (5-8 stalks?) but now that I'm back on, even yesterday my first day back, I would eat a couple slices of cucumbers, or a few bites of celery and be full for a while. It's pretty amazing how fast your body goes back into medifast mode.. almost like it really enjoyed being there (more than your mind did.)
Sorry to turn this into all about me. I just wanted to let you know that there are others in the same boat as you