They're wearing me out

I've been teaching these three-year-olds for a week and a half and I'm worn out. I have a very busy bunch of kids. Trying to get them to all sit on the floor and listen has been a challenge. They are rolling all over the floor, sitting on other kids, hiding under the table. Some days I just don't know what to do. My assistant has been a huge help. I don't get it - why do they listen to her and not me. There have been a few days when I've cried on the drive home. I want to make school an enjoyable and exciting place to be, but I feel like I can't do a lot of things because I'm trying to discipline all day. I've been told that if I just hang in there things will get better. I hope it's soon. They have been getting better since the first day, so there is hope of sunny skies in the forcast.