How much have I messed my kids up?

I want my children to reason like adults, and this is causing my entire family endless stress. One of them is full of attitude and contempt right now, and trying purposefully to upset me, so that from sun up to sun down, I must be in battle mode to make it through the day. She lobs bombs and shoots gunfire my way, and will sneak in at least a half dozen ambush attacks where she starts a normal conversation or shows a sweetness, only to lure me in before turning the whole scene into a mess of negativity.

And when this happens, I first react calmly. I tell her what she must do and I make her do it. But eventually, I lose it, and I shout. Sometimes I bully her into doing what she should be doing. The next phase is reasoning and explaining, where I tell her my side, then ask her about her side, and try to figure out, or get her to figure out, what her deal is.

None of this works, obviously.

As parents, we're supposed to be calm, inflappable, upholding the rules because they are rules. Not letting emotions in. Not making it a bigger deal than it is. Either put your pants on and go to camp, or don't and stay home. Why isn't it that easy when I'm not typing? One is that I have twins, and I can't just keep punishing one for the other's behavior, but it's more than that. I want them to do the right thing because it's the right thing. I want them to be able to tell when it's wrong and bring good, solid points to the table as to why it should be another way. I want them to feel like their feelings are valid.

I do not want them to call me stupid, throw things at me, or willfully disobey me.

But I've got both.

And what's messing me up is myself. I'm SCARED I'm messing them up. If one is acting out because I'm spoiling her and letting her do what she wants whenever, then I have to crack down.

But if she's acting out because she's craving loving attention from me (and she DOES need a lot of loving attention which she has been foregoing to do this crap), then cracking down on her will only prove to her that I don't love her? But should she need such explicit validation all the time? But if I stop hearing her out, will she decide she's totally unimportant?

How much have I messed my kids up, is the question, to be honest. And what the hell do I do.