Adult way to handle this 🙃

I 😔😔😔🙃🙃🙃🙃🙃🙃🙃🙃

was offered a job promotion, for shift leader, I said yes. but im realizing I might not be able to handle the job and the job is going to be too hard. I'm 22 years old. I want to be mature about the situation. What's the adult what to handle it?