My first two days were really tough the first time, and I think it had a lot to do with the way I prepared my body. This time, I was very good about what I ate and drank for the week beforehand, and this cleanse so far has been a breeze. I'm on Day 5 now, and my focus and energy are great. I can't say for sure that these two things are cause and effect, but that's the way I'm leaning right now.
However, the first time, my Day 3 was great, so just hang in there and let your body flush out for a couple days!
You also might try a simple meditation. Try to sit someplace with filtered light and let your thoughts go, since you can't focus anyway, it should be pretty easy :)