Help! I just cannot control myself

I relpased today, and tomorrow will be my mid-term. Now I feel terrible.

My problem is that the urge just attacked over and over again. In the first few times today, I successfully distracted myself from it through focusing on my homework, singing a song, and eating something. But the urge just didn't let go of me. Every time I resisted it, it just come back a few minutes later, and even stronger. Finally, an hour ago the urge just grew so overwhelming that I surrendered.

Do you ever such experience? I'd like to hear your advice. Next month will be my AP exam and I really don't want to mess up with it.