Self-acceptance - the acceptance of the person we are, with whatever defects and flaws - imperfections - and even our perfections, determines who we are and who we become. How can we work towards becoming anything else if we don't even know who we are now? How can you chart a course from A to B if you don't even know where A is, or where B is in relation to you? If you don't even know what your own strengths and weaknesses - and character flaws are - how can you know what to work on, and where to improve, and what to leave behind?
Whatever we are now, we all began as something else. Personally, I'm far more partial to the view that "I am what I choose to accept I am", and "I am what I want to be".
I am what I am. Nice sentiment that. But is it that simple?