Congratulations!
Don't sweat it, just absorb the experience and do your best. If you get fired, the world won't end. For now, don't worry about how it might end, just focus on learning to do it well.
Jobs can end for all sorts of reasons, many having nothing to do with an individual's performance. Sometimes the company changes ownership, or has to downsize for fiscal reasons. Sometimes you end up working with a manager who's just a d*ck. Still, every job is experience that you can pay forward into the next job. Its all useful, even when its not pleasant. But who knows, you may love the first one, and end up owning the joint someday. Good luck!
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"Strange, inaccessible worlds exist at our very elbows"
- Howard Phillips Lovecraft