(Currently I am on day ten of a water fast. I will go much longer...)

I know this may not be much consolation, but the greatest motivation is the deep cleanse that you will only experience if you take it longer.
You just have to trust that the reward is waiting for you, whether it is physical, spiritual, or both.
As you have only gone three days before, you cannot have experienced this.

I can only tell you from my experience, which is after three days, I am still hungry, grouchy, and I have the worst case of cotton mouth in the world, as I have only just begun to detox. It all does depend on many factors, such as how healthy you are, and how much excess weight there is on your body. The more weight, the more fat cells, and as fat cells store all the filth our body accumulates, the more toxicity, and the longer it should take to cleanse sufficiently. For me, when my tongue clears up, I know I am finally getting somewhere.

Now, I don't know why you are fasting, but if you want to lose a lot of weight, and you are expecting a big result from a seven day fast, you can at best expect to lose fourteen pounds, which is likely to come right back if you don't both break the fast correctly AND, drastically change your eating habits. If weight lose is the issue, I would suggest a much longer fast, not on water, but on fresh squeezed fruit or vegetable juices. Also, if you have health problems, I would only do an extended fast the first time under the initial care of a holistic M.D. It is really easy to fast 21 days or more on juices, and actually have as much or more energy as you did when you were eating, so it won't affect your ability to work and play.

As someone who has done several one week water fasts, and many three week juice fasts, I can only tell you that the longer the fast, the more likely you are to completely cleanse the body, change those habits, and actually find yourself craving healthy foods like raw fruits and vegetables.

Those new eating habits are far more important than the fast itself, and equally important is how you break the fast. There is a lot of information on the web, and as I am not a doctor, I will not tell you how to do this, but read a ton, and you will find the wisdom that is common to all approaches.

Good Luck!