“The best preparation for tomorrow is doing your best today” - H. Jackson Brown, Jr.
One of the peculiar things about how life on earth works is that we can only live one day at a time. We can't skip ahead and live tomorrow before today, and we can't go back and change the way we lived yesterday.
Nevertheless, we often spend so much time worrying about the future and the past that we become paralyzed, or at the very least stop being intentional about the present (which, we may recall, cannot be done over again once it's past).
The only part of our lives that is actionable is what we have right here, right now, so the best way - the only way - to take control of your life is to be intentional about what you do in the present.
Not only is doing your best today the best preparation for tomorrow, it's also the best preparation for yesterday. Because of the constantly-moving nature of time, all you really have to worry about is what to do right now.