Seeing is believing. When you see something's possible then you can believe you can do it. If you're trying to land a new dunk a lot of times you have a lot of close attempts. As soon as you land one, it seems to open the flood gates and now you make it all the time. Now you KNOW you can do it. You 100% believe its possible because you've already done it.
Instead of waiting till you achieve it to believe in yourself, BELIEVE FIRST! and see yourself achieve all your goals.