Do we accept human nature, or do we fight to improve ourselves?

I think we do both. We have to accept ourselves for who we are before we know what needs improvement. We also have to accept others as they are, realizing that we may have once been where they are. Nobody's perfect and aiming for perfection often leads to failure. I think what helps is in aiming to surround yourself with good people, so you have models and examples of what to aim for. If you surround yourself with people who don't have these kinds of goals, chances are your life too will be fraught with stagnation and decline rather than personal progress, accomplishment and fulfillment.