So how come I am not the mommy of my fantasies?

I mean, we all had them, right? The fantasies of what we would be like when we were finally mommies? i know when I used to see parents yell at their kids when I was younger, I was so
smug in the certainty - the absolute certainty - that I would never do that. I would always be in control, I would be calm, I would be patient, etc.
Don't get me wrong. The kids are pretty well behaved compared to alot of th e kids I see around them . They really are. But I still get pretty frustrated with them. Just put them in bed,
after listening to them yell and yell and fight with each other, and I got annoyed and I yelled and then I felt guilty and here we go, downward spiral, round and round. All the other mothers seem so calm, and I feel.. so frantic and uncalm.