I understand, completely. i joined 3FC a few years ago, and I was doing such a good job at losing weight, I was even nearing "Onederland" for the first time since I was about 13 or 14! And then I just kinda let things go. I stopped logging what I was eating, I stopped exercising (but lets face it I never did that much anyway) I went from a retail job where I was on my feet for 40 plus hours a week to a desk job, I started dating a guy and picked up some of his eating habits and now we're married and after all of that in the past year and a half, I'm 70 pounds heavier and the shame is overwhelming.
You're not alone. It's happened to a number of us. Hang in there. The fact that you're here again, posting about it and trying to get back on track is an accomplishment in itself. I sat around for months telling myself I needed to get back on track before I logged back on here. And like you said, you have done it before, you can do it again! =) Good luck!