Best Isn't Always 100%
Give your best in everything, whatever your "best" is at the moment.
We have varying degrees of mental capacity and energy at different times. Sometimes we're pumped, high on motivation and ambition. Sometimes, it dips low, especially after a rough day.
If you're at 30%, then your best is 30%. You can't possibly give more, without borrowing from your health—physically, mentally, emotionally. Your best isn't 100% everyday, every hour. Be at peace with that.
Just do your best at that moment.