I've felt like this for a long time, and it can be stressful, but it's just a part of growing up. Sometimes I forget I'm still doing that. I think we all are. It's really just fear of the unknown, because I'm being faced with making big decisions with some really big consequences that involve doing things and going places that I haven't done or gone before, and doing it by myself. I think the by myself part scares me more than anything else.
I'm scared of making the wrong decision. I'm scared of being by myself in the middle of nowhere with no friends. I'm scared of being stuck in a dead-end job and writing never taking off. I'm scared people think I'm totally lame.
This is where you have to let your mind take some control over whatever crazy emotional chemicals and hormones are flooding it. Yes, these things are scary, but they're not that scary, and we just have to decide what's worth it and not let fears get in the way.
Not only is that real life, but it's what drives a good story. At the heart of it, the protagonist is either trying to escape their fears (a serial killer or evil stepmother) or trying to conquer them (a dark wizard or fiery eyeball trying to take over the world.) And the climax is them in the same room with that fear, looking it right in the eye. Sometimes literally.
I've come to the conclusion that fear is okay. Letting it overpower you is like being in a tunnel and denying the light at the end of it. Even if right now you can't see it, it is always there. Stories remind us of that.