Work Anxiety

I start my new job tomorrow, haven't worked in the past 4 years due to college. The first day I'm going to be trained, but my boss wants me to come in on thursday and friday and work ALONE, by myself! after just one day of training. I'm freaking out and so many scenarios are playing through my head, what if a customer comes in and i don't know how to awnser their questions? I feel so uncomfortable about the situation and don't know if i can handle it, I'm going to try but i feel so tense and stressed just thinking about it. Has anyone else here experienced something like this? i would love to hear your story.