DAE feel like they weren't mean to "grow up"?

I'm over 18 and I'm in a situation when I need to start working and all that, but I simply feel like I don't have the confidence or competence for it. I don't consider myself an "immature person" in personality, but there's something in me that just makes me feel like I wasn't meant to grow up because after age 15 I definetly started to lose my abilities and I feel like I just get less and less confident each year. Obviously I have some trauma history that explains that behaviour but aside from that, I wonder if other people here that feel the same way....What are your thoughts?