Dear devilfrancis,
It is natural for you to feel this way. However, you can do something even more positive than wallowing in regret. An important part of acknowledging your wrongdoing is to sincerely repent and make an effort not to commit the same mistakes/wrongdoing in future.
Kamma is not a simple equation of 1 + 1 = 2. The workings of kamma are too complicated for me to explain it here in a few sentences.
But here is an excerpt from the Buddha's teachings for you to ponder upon (extracted from
here):
"Now suppose a man throws a lump of salt into a small cup of water. What do you think, monks: would that small quantity of water in the cup become salty and undrinkable through that lump of salt?" — "It would, Lord." — "And why so?" — "The water in the cup is so little that a lump of salt can make it salty and undrinkable." — "But suppose, monks, that lump of salt is thrown into the river Ganges. Would it make the river Ganges salty and undrinkable?" — "Certainly not, Lord." — "And why not?" — "Great, Lord, is the mass of water in the Ganges. It will not become salty and undrinkable by a lump of salt."
*Salt refers to Kamma
It does not do one any good to keep pondering upon past wrongdoings. It is unproductive and does not help you to heal. Focus on doing more good deeds instead such as meditation, chanting, volunteer work etc, you will begin to feel less guilty.
I truly believe that you have other good qualities. Look at them instead of focusing on your moment of wrongdoing. Sometimes, we tend to concentrate too much on our bad qualities and feel depressed because of them. However, being aware of our bad qualities give us the motivation to strive for improvements. One who is unaware of one's bad qualities does not see the need to improve oneself.
I hope that with my limited experience and knowledge of Buddhism, I have managed to help reduce your suffering. May you always have happiness and be free from suffering. May you achieve your desired results for your examinations.