Self confidence is about having positive energy and over confidence is about negative energy. Let me explain this. You have prepared well and revised your course well before the exam. Then you are confident of writing the paper well. This is self confidence. Over confidence is when you top the exam last year and now you are sure that no one can beat you even if you don't study well. Now, when the results are out you get lowest marks because of not preparing well.
People sometimes tend to be over confident when they get success by applying very little or less pressure. They feel good that they were able to accomplish the task with minimal efforts. So, when they are required to do the task again they feel why should I work hard when I can get success by applying less efforts.
Hence, in order to be self confident only key is Practice, Practice and Practice. Utilize your time preciously!