To the extent that a man's hand is real, & setting aside death and violence, then to that extent he may use his hand as he chooses.
So also with self-acceptance. If you decide you are inadequate, or choose to believe so, that judgment will result either in self-castigation to no useful purpose or (at the other extreme) to spurring oneself on to improvement & accomplishment. If you simply ignore judgment (the notion of better or worse) and say to yourself,
"I'm my self and that's all; nothing will change who I am, so it's unreasonable that I should try to be that which by nature I am not,"
then you are abdicating self-guidance, abdicating the ruling faculty, even abdicating oikeioosis (self-interest), the fundamental rule of dynamic nature. It may be that you can never truly & fundamentally change who and how you are, but at least you can learn and improve and often - in many cases - excel in relation to your original self or others.
To refuse to accept your condition, your essential given - whether a world of causes you have no control over, or a self you cannot exchange for another - is in either case to condemn yourself to despair or self-hatred. To simply and passively accept conditions is, however, to abdicate what little freedom you can exercise and diminish yourself altogether. The untidy answer is to do both - accept the world or yourself as the starting point ("warts and all") and then proceed towards the Good therefrom.