There is a huge difference between "striving to improve" and "being dissatisfied with what you are", and the difference lies in your attitude. You can be positive about who you are and still wish to make yourself better. It does little good to be satisfied with who you are, because - as you say the words - "who you are" turns into "who you were".

We are not objects as much as we are processes. We are always becoming one thing and ceasing to be something else. Managing those changes is a full-time job. What we breathe, eat, drink, think, etc. effects the process one way or the other. This is why we need to keep paying attention to these things and not take them for granted as much as we do. After all, it doesn't take much to change the process from positive to negative, or vice versa.