I found when I used to get colds, which was rare, the thing you need to do, if it's possible, is to stay in bed and sleep. I've found that if you have a cold that won't go away, if you stay in bed and sleep for a long period of time, it will give your body the chance to fend off the cold. If you can, try staying in bed for one full day, and while you're in bed, try to sleep as much as you can.
That one full day should make the cold go away.
