I think being best might mean aligning with what you have done and having no problems with that.

Knowing from within what you want, or a type of thing you *might* want (for instance success, or money or a better life) and knowing that maybe today has been a day for a good improvement and being done that, or having done slight bit of improvement, or maybe non because of laziness or an excuse. But knowing you were not able to do what you wanted to do for whatever reason, you can make an effort to make it up for it the next day.

I think being your best includes that. Being true to what you want, trying to achieve it while taking responsibility of your own capabilities and shortcomings. Being true to your mind, sincere to your goals.