I have found this to be SO TRUE as well. There are times that I just couldn't stay with the Cleanse more than 1-2 days. And when it is right, I know it and can perservere.
I found that when I was really in the mindset to stay on the Cleanse, I could ignore all the sabatoge in the world around me (food ads, lunch and dinner invitations, inviting smells of food, unsupportive friends) and kept my mind focused ONLY on my plan (and success).
Maybe you should really look at your reasons for doing the Cleanse at this time?