Belief is the foundation of any success.
For any goal that you are pursuing, you need to believe in yourself. Only by believing in yourself will you put the right amount of energy that will make you move towards your end goal, whichever it is.
It is very easy to doubt and wonder whether we will make it if the task at hand appears too big. If you are in this situation, my advice to you is to take a step back, and write down the different actions you need to follow from now on to your end goal. By taking a task at a time, it will make it easier for your mind to be convinced that you can make it, and will make it easier for you to stick to your plan.