It's hard because it is an extremely busy atmosphere and everyone is rushing to get things done. This makes it harder for employees who have been there a while to have sufficient time to train new employees. As long as you keep trying and keep helping as much as you can, you should be fine. Also, keep in mind that you probably are not in the way, it just feels like you are. Once you get adjusted, you should be fine.
It isn't hard...in fact I've often seen some retarded people who work in fast food places. Ask your manager for advice on how to be better at your job.