Wow, you sounds just like me. The exact reasons why I chose to do this fast. And God has been calling me to a healthier lifestyle for some time now.
My suggestion is to remember: " I can do ALL things through Christ who gives me strenth." If you quit it is as if you are admitting that you are weak and suseptible to temptation.
Which of course all of us are. Don't get me wrong with as many successful fasts that I have had I have had just as many one or two day quitters. If you struggle just keep going. Try again. It will happen and your resolve will harden.
But most of all, PRAY! It will keep your mind focused.
Okay, I am sounding preachy so I will just say: GOOD LUCK!