Be willing to let go of being good to become great. There is so much meaning in this quote the more you look at it.
One of the first things that comes to my mind is that you can't hold on to something that's good enough and expect something great to show up.
One of the Universal Laws is the Law of Vacuum. In order for something to show up in your life you must create a space for it, a vacuum. When you're willing to let go of what you have to make room for something better, that's when greatness can come into your life.
Otherwise you could work really hard trying to gt to the next level and see no progress because your cup (your life) is already full. There's no room for anything else to be poured into your life. You've already filled up all the space with things that aren't exactly what you want.
You must be brave enough to let go if you want to live an ageless lifestyle. It's all about stepping into the power of who you really are.