Don't forget that lots of "normal" people do stupid things that make working hard for themselves. How many times have you heard the old story from the guy next to you. "I didn't sleep at all, I was at the bar all night." Or. "Oh man, (video game) released at midnight, I've been playing it all night, why do I do this?"
Don't forget the sheer volume of places that have employees that are just sodding lazy. Yet have worked there for years.
Everyone has bad days. It happens. If your boss wants to fire you for having one bad day. He's an asshole you don't want to be stuck working under anyway.
As for the interview. Yup, nerve wracking. You are definitely not alone in that fear. Most everyone gets nervous about a job interview. Try to remember that all you can do is your best. Deep breath, eye contact, firm handshake and be honest. You will either get the job, or you won't. Not the end of the world.
What I am reading from you, is someone who has skill and a strong work ethic. Two things that are often sorely lacking these days, from people in most every occupation. I don't think you have nearly as much to worry about as you think you do.