Don't let yourself think about yesterday. When thoughts of yesterday come to mind, acknowledge them but state that today is a new day and you are prepared.

Take a few minutes before the exam to have a quiet walk. Talk to yourelf about staying calm and focus on your breathing to make sure its long and steady not short and quick.

Eat some fruit and drink water before going in. If they let you have water in the exam then sip on in during the exam to keep your mouth wet and to remind you to breath.

Just some off the cuff ideas for you to try. If you start to get distracted with racing thoughts, acknowledge them but then refocus yourself again on the work at hand remember to breath.

Hoping things go better today for you. Then find someone in or out of the college to talk to about stress and anxiety management. There are some good resources here on PC too. A great self help manual that has a section on anxiety that is helpful. I'll try to post the link here to save you hunting around.