RedMist, so much of what you say could have been written by me. Not the house - I have low standards there - but my DS 9 and DD 5 drive me insane. I got to the point with DS that I yanked a handlful of his hair and physically pushed him out of the house, saying he had to disappear for 10 minutes for his own safety. After I'd calmed down I was devastated and ashamed, but recognised that we could get there again. (I was already successfully on 10 mg of cipralex - an AD - a day, so it wasn't depression.)
There's been lots of good advice on here, lots of which I look forward to using. What I did in my circumstances was write a list of rules and went through them with the children. They are stuck on our fridge and in the children's bedrooms. It helped on lots of levels, DS understood, DD sort of got the idea, I understood that before then although I knew what the rules were, nobody had articulated them to the children, and best of all I reminded myself that I was the grown up and had to behave like one. It helps me to keep a grip. Just in case they might be of use to anyone else, here are our rules (very sorry it's so long):
GENERAL PRINCIPLES
? The grown ups are in charge. The grown ups love the children and want what?s best for them. The children do not always understand this or believe it but that doesn?t matter.
? Fun stuff (including playdates, parties, TV, computer, PSP, football, rugby etc and anything else the children enjoy) are treats, not entitlements.
? We have family rules. They are for everyone?s benefit and everyone has to live by them.
? If the rules are broken then there will be consequences and treats may get taken away, at the discretion of the grown ups.
? The grown ups can make new rules or change the rules whenever they want.
THE RULES
 We respect the grown ups, who are in charge. We listen to them when they are talking, and we respond appropriately. We do as we are told without complaining. We do not answer back.
 We do not behave violently or aggressively. This includes hitting, punching, kicking, pushing, screaming, yelling etc and also threatening behaviour.
 We use good manners. We ask nicely for things and we thank people when they have done something for us. We sit at the table and eat nicely at mealtimes. We wait until it is our turn to speak. We do not interrupt each other and especially we do not interrupt someone on the telephone.
 We communicate nicely and use nice language. We do not swear or say rude things or call each other names. We do not shout. (Mummy will find this one tough.) We talk in a pleasant tone of voice.
 We treat each other kindly. We care about each other?s feelings. We are not mean or unkind to or about each other. We look after each other and try to make sure that everyone is happy. We do not touch each other?s things without permission.
 We do not whinge. If we have something to say, we say it in a big grown up voice. Especially, we do not whinge about going in to school.
 We do not lie. Especially, we do not lie about our behaviour and whether we have broken The Rules. Lying is worse than just breaking the rules in the first place and it makes grown ups furious.
 We take responsibility for our behaviour. We understand that there are consequences for breaking The Rules. If we do break The Rules, we accept the consequences without lying or complaining (though we are welcome to negotiate in a grown up manner).
 We understand that although other people annoy us sometimes, we probably annoy them too. In a family, everyone has to put up with everyone else and get along together.
 The grown ups are in charge.