Mradul, it is a tough world that we live in and sometimes we are tough with ourselves. Agreed that time once gone can never be restored. So does the time right now where in you are thinking that you have wasted 3 years and yet have achieved nothing. You cannot change your past and you cannot predict your future. But what you have in your control is your today and that is what can help you build your tomorrow. If wasting time is end of life, world would have stopped right after Adam and Eve ate the fruit of Wisdom. But no. Life doesn't end there. Instead it starts there.

We do not know what might be your situation or the reason why you have wasted 3 years but we can assure you that nothing can bring those 3 years back. But regretting about wasting those 3 years is not going you as well. Building confidence is like fitting pieces into a puzzle. It doesn't happen over a day or a whisk of a second. It is a continuous process of evolution of your inner strength. May be you should write down your strengths and weaknesses. Also write down the Opportunities that you have that you can see for yourself and the immediate threats like low confidence etc. For a certain period, forget that you have weaknesses. Focus on your strengths and opportunities that you have at hand. But at the back of the head remember that one of your greatest strenghts is that you have acknowledged that you wasted 3 years and the opportunity is that you are willing to make amends. Tell yourself every day atleast one memory from your past when you were strong and confident. A confident incident when you  aced somethng that you thought you can never. It might be as simple as solving a sudoku puzzle to as big as riding a 2 wheeler for the first time. Go through the memory lane and imagine how you felt at that very moment. Close your eyes while you do that. And once you are done with that incident, imagine you setting into the future with a similar instance where you are braving something against all fears. Like may be flying a plane. Imagine things that you wanted to but you couldnt. Imagine you doing them very easily. Feel those moments. And now always tell yourself that you can survive no matter what the problem or issue at hand might be. Self talk is the best way to build up your confidence.