What types of foods will help me counterbalance the stress and burden of daily strenuous exercise?

I signed up for a 30 day challenge, which has started taking a toll on my energy and vitality. I feel so depleted at night time, that I'm unable to focus my mind on tasks I want to accomplish. Rather than pumping me up and invigorating my mind and spirit, it's starting to make me feel deflated and lethargic and irritable. Perhaps I could counterbalance the stress of this exercise with a better eating regime?! I don't want to give up now, I have only 6 more days to go, and I do ultimately want to become fitter and stronger and capable of withstanding such pressures.