It's not just fear. It's fear of the unknown. You admit this is your 1st ever job, so it can be hard to calibrate expectations from employment. It's not all great though, and even at a great company, you just might be in the wrong place or working on the wrong product.
I think personal fulfillment is key to good development as a person. You say you're unfulfilled. Why? Can you make a list? Where do you think things might be different?
I'd also encourage you just to look for a job, and maybe take some interviews (perhaps at startups you think are in an interesting field). This might give you some more perspective, and either let you realize that you want to do something else - or let you recognize how good you have it, and that you shouldn't be taking it for granted. Also, recognize the difference between the perception of starting your own business vs actually doing it. It's a lot of work. It can be great, but it is certainly easier to jump on a small company that is doing something you think is interesting.
You don't know what you've got till it's gone. But you will have many jobs in your life, so I'd just suggest you not get too hung up on any one particular job, and view it more as a long journey of exploration.