This powerful quote speaks about a core principle of our reality that many overlook or are unaware of: We shape our limits.
To briefly explain this concept, no human being is remotely capable of truly understanding his potential, we simply can't know how much we can grow.
If you read the biography of high achievers like Albert Einstein, Benjamin Franklin or Richard Feynman, you will see that they never had any clue on the change they would make in the world, they were simply committed to never stop learning and improving as individuals.
This mindset defines successful people, if you never try to assume how far you can go, there isn't a place you can't reach.
So if you see yourself thinking of things like "I will never be happy/smart/successful or have great relationships and etc" take a deep breath and say to yourself "I simply can't presume that I know the future, no one can, and I may have the potential to be way more than I ever envisioned for myself".
With time, you will perceive how much you can add to your life by simply forgetting the concept of limits and breaking barriers on your personal life will be a daily occurrence.