I went through this process in my therapy, from feeling like I was worthless, to now feeling like I have some value. My old therapist and I did not see eye to eye on what contributes to value - he says people have value for what they do, I say people have value for who they are.

So you can go at it from either of those angles and see if either one resonates with you. Think of things you are good at, and think of your beliefs, values and principles and if you feel that you live your life in accordance with those values.