Stressed out and tired

Things are not going brilliantly in my new job. Made a few mistakes in the last couple of days...some of which the boss was really pissed about. The previous girl was working full time, Im only 4 days a week and yet I am expected to do the same amount as she did and in one of the busiest times ever. I am stressed all the time, trying to get stuff done. I woke up at 2am, Heart racing and just feeling like crap. I wonder if I am truly cut out for this shit.
I also feel as if I am not putting enough time and energy into the kids, into Steve and into living.....Stop the bus, I want to get off.
And to steve..this has nothing to do with the supermarket stuff, its purely the new job and the amount of work on there and the speed at which I am expected to do everything, expectations and just plain everything. Its a struggle.