Remember some amount of doubt can always be helpful in gaining wisdom or achieving advancement in life. But when it becomes a cause for your depression and inactivity or when it stands like an insurmountable hurdle in your way to reach your destination, draw upon your energy reserves that can toughen up your mind. You have to strengthen your will to succeed at all costs and weaken doubt by all possible means, so that you will lead a life of fulfillment.
You may succeed because of your doubt or in spite of your doubt. Or, you may have to accept the inevitable and compromise with whatever worst-case scenario that presents itself. If this happens, just change course, re-energize your cylinders and start afresh on your revised path. Defeat doubt before it defeats you.