I have a lot of the same thoughts about myself. Although I still struggle with it, two concepts have helped me. One is that I can do good for someone, but not unless I take care of myself and get myself in a position to be able to have something to give. You have to secure your own oxegen mask first, then help someone else, right? The second is that I believe that God created me and sent me here for a purpose. As tempting as it is to consider myself an afterthought and less valuable than His other creations, I don't want to insult Him by denying what He created and gave to me.