I once heard a great question on the well known chazal that there are 3 books open on rosh hashana. Tzadikim gemurim are inscribed for life. Reshaim for the opposite. And for beinonim who have the same amount of mitzvos as aveiros they wait until yom kippur and are reassessed; if they’ve done teshuva they get life otherwise cv not.
But why is the only option given to them that they must do teshuva? Surely if they have the same number of mitzvos as aveiros then there is another option open to them: to do one mitzva! They will then have more mitzvos than aveiros and will be called tzadikim. This way is not brought down. Why?
A: We are all judged from rosh hashana to rosh hashana. So doing another mitzva after rosh hashana will not help as that will only be added to the new year’s count. It cant change last year’s cheshbon of equal mitzvos to aveiros. The only thing to change that is teshuva with charata on the aveiros of the past year. This will pardon and annul some of the aveiros leaving the past year’s balance with more mitzvos than aveiros!