You are not alone S., I have a 10 yr old and a 2 yr old, and I've been trying (for like 10 years lol) to do the same thing. I think I've gotten a little more patient as I've gotten older, but something that I read somewhere helps me out when I'm talking to the kids (or getting ready to pull my hair out!) I don't know where I saw it, but it was something about getting your kids to cooperate and keeping your cool. It was something like this-- Imagine your boss or your spouse talked to you the way you talk to your kids. Instead of saying "Could you get that report to me by noon?" they screamed "Give me that report now!!" Or instead of "Would you mind doing the dishes tonight, I'm so tired," it was "If I have to tell you to do those dishes again I will lose my mind!" The article said that's it's hard to remember that "treat others the way you wish to be treated" also applies to our kids. If we want them to listen and not give us attitude, we have to treat them with respect, the same way we want other people talking to us.
I don't know if that will help anyone else or not, but although it hasn't cured me of my tendency to scream, it has definitely helped. I try to phrase things in a more calm and nicer way, even when I'm upset. Something about saying please actually seems to calm me down and remember that it's just a little thing and I can still be nice about it.
It kills me when I lose it with the boys, but sometimes we're just overloaded and they are too, and that's a combination for disaster :)
I hope this helps a little, it's still a struggle for me but that article really helped me see things in a new light.
Best of luck with everything!
J.