Don't be dejected because you couldn't get past later in the day when you gave up. It is called intermittant fasting, and some think it's is as healthy if not more so than the extended fasts.
However if you unsist on doing the long fast, I can show you how to succeed. EFT is the answer. [Emotional Freedom Technique].
https://www.youtube.com/watch?v=UnTwiQY2hcM This relates to getting rid of emotional problems, but it works for anything.
I use it for fasting, pain, or anything else I can think of. All it is basically is accupressure points that allows you to access the mind that lets you change whatever it is you want to change.
Now once you learn the tapping points, ETC.. If you start to get hungry tap while you think, and visualize not eating. Try it.
The best way I have found for fasting is start a few days before you want to start the fast, [Maybe a week].
Tell yourself that on such and such a day you are going to water fast, for X number of days. Repeat this several times a day.
While in the fast if you start thinking of giving up, tap that you are going to continue the fast until you have done what you started out to do.