It isn't useful to tell somebody to do their best. I've found in my experience that most of the time, this phrase gets trotted out when the person on the receiving end is already discouraged or frustrated.

Nowadays I'm trying a different phrase, both with my son as he works through his schoolwork at home and with myself as I continue to navigate life and all of its challenges.

You wanna be your best self? Stop it with 'Do your best' and try this instead.