It could just be that your positive qualities are there, but buried underneath years and years of judgment from people who do not understand ADD. I’m actually dealing with the same problem. I got so used to being labeled negatively at school, work, and the people around me. It made me doubt my own worth. I know it’s hard to stay positive and self-confident in a world where we are expected to think, function, and succeed in the same way, or else we get underestimated.
You need to do some soul-searching and ask yourself what skills you have that make you confident. Also ask yourself what people seem to like about you. Are you a good listener? Are you super-outgoing? Can you draw/write/paint? What makes you feel confident about yourself? Whatever your strengths are, they are valuable and it may take you some time to figure out what you can offer the world.
I believe that the ignorance of society has shattered your self-esteem just like mine was. The problem is not you; it’s the lack of awareness about what we struggle with and how we process what we learn. People don’t understand why we don’t fit into the same cookie cutter, so it leaves us feeling discouraged and hopeless.