well don't bother doing anything tonight, it's too late in the day and obviously you don't feel up to it anyway. Start afresh tomorrow. Write down targets for how much work you want to get done in each day leading up to the exam. Make the targets small initially so that they don't seem daunting but gradually build them up. Make sure that when you are revising, you have no distractions, go somewhere quiet, turn your phone off, if you have to use a computer to revise, close all forums down and don't go on MSN or anything that will distract from your work.
Make sure you have sufficient sleep and that you have eaten well and take regular breaks to maintain levels of concentration but know when to return from the break. Don't let a 5 minute break become half an hour. You have to be self-disciplined. It's in your hands and no one elses.