It means that when you come to a point where you are questioning your ability to do something, remember everythung you have already done. Rememeber your first fear and how you got over them. It always seems tough at first, but you will and can get it over it.
Personally, I agree with this quote because its easy to get hung up on your failures. however, if you look back on everythung you've done, you can keep moving forward.