Getting nervous at crunch point

I have always been a hard working guy. I do give enough time to prepare things up. Even when i have prepared hard for any occasion i dont find myself able to perform well enough. With the time closing in i get nervous and with that continously rising nervousness i get apprehensive inside. After tha i try to escape the situation. Many of time i escape without performing the task well enough for what i have spent days and month preparing.
Bad habit!
I know but i cant help. I am now trying to get rid of this problem and facing them with keeping in mind that this is not the end this is the beginning of what i am born for.