I know precisely where you're coming from as well, techgirl. I've cheated and stopped and started and stopped and started so many times it's ridiculous. The problem with me is that I'm impatient and want to see quick results and quick changes. It is very hard to get back on track but I know that, for me, the first three or four days on the diet are the big test. I think that if you can get through them, you've past the hardest part and are well on the road to success. The difficulty is, of course, restraining from cheating during those first days. Once the cravings start disappearing, the hunger level drops, I begin seeing differences in my body and on the scales, I really become motivated. Coming on here and reading about people's success is also very inspirational for me. Check out this site for tips and encouragement whenever you feel like you're on shaky ground, it's what I do and it always makes me feel more determined.
As Alto pointed out, are you sure that you're on the best low-carb plan for you? It took me some experimenting with the Atkins Diet, Protein Power, and the Zone before I discovered that Atkins was definitely the plan that works best for me. Have you read up on or tried some of the other plans?