Lacking self-confidence

I am an engineer by profession. I work very hard and Alhamdulillah seem to accomplish whatever I put my hand into. But above all, the problem I face is the continuous fear and lack of self-confidence before starting any task while at work. I even know that I would be able to accomplish it, but
don't know why, get constantly reminded by the sub-conscious that I won't be able to make it. Due to that, I constantly face a feeling of reluctance
before starting any challenging task. I am capable enough but don't know why this fear of starting any task has been developed in me ever since I
graduated and stepped into professional life (approximately 1.5 years).