> I'm sure they had tons of better/experienced people to hire
Maybe they saw potential in you. Maybe you’re cheap. Maybe you’re good at interviewing. Maybe they’re bad at hiring. Whatever the case is, they picked you so don’t obsess over reasoning you can’t possibly figure out.
> What if I don't live up to their expectations?
You’ll eventually get fired or you’ll get so miserable that you quit. But most companies don’t like firing people so they’ll give you second, third, fourth chances, etc.
So ask for feedback. Ask your manager (not too often) what to improve on. Take positive feedback as positive feedback. Don’t obsess over whether they’re keeping something from you.
> I've always had trouble drawing the line between work and personal life, how do I balance this
Is this a remote job? If so, only you can make yourself stop working. In that case, you have to develop discipline. If it’s an office job, get in on time and refuse to stay after a certain time. Make up a lie about taking care of a relative or something if you have to.