How can someone who thinks negatively most of the time, bein to think positively?

I tend to doubt myself in a lot of things and I noticed that it does have an effect on me for example when im doing school work and I don't know how to do a certain problem I get nervous and feel a tight pull in my stomach and I start to tell myself "man you'll never be good at this" I really don't want to do this anymore, because I know it's holding me back from reaching the greatness I know im destined for. So if someone has any advice on how I can begin to think more positively about things I really look foward to your answers. Thanks.