"How do I let go and acknowledge that my almost-three-year-old is programmed to be incredibly challenging - as I suppose they all are at this age - without losing my mind or letting her rule the house? Help. I'm in an endless loop of setting limits, enforcing limits, and getting mad."
Wow, have we all been there. Except for those of us who haven't been there yet but will be. Before I get to my half-answer and ask for everyone else's suggestions, I want to point out two things that might help frame this whole testing-limits phase (which seems to last from around 2 1/2 to right after 5 for many kids):
1. Many many many kids go through this, so it has nothing to do with how effective you are as a parent that your kid is testing limits. It's a combo of their personality (and honestly, being a tester is probably going to help them in later life) and how you've structured things, and having structured things so your kid feels safe testing boundaries and your love is not at all a bad thing.
2. Your child is not going to end up spoiled or ruined or out of control if every moment at this age is not a teachable moment or if you just give in sometimes, or even a lot. As long as you can maintain a basic level of trust between you, not enforcing each boundary all the time isn't going to turn your kid into Lindsey Lohan, so choosing harmony over policy enforcement is a completely valid course of action. Just check yourself to make sure you're not creating a feedback loop that increases tension because you're inadvertantly rewarding bad behavior.
2 1/2. A lot of kids at around 3 1/2 are such little cauldrons that even if you do completely give in and let them steamroll you they will still make a tantrum out of your giving in. If you can't win anyway, then just doing it "because I said so" is an awesome policy.
And now for my half-answer: Having had two kids make it through these years with all of us still alive, I am convinced that how difficult this stage is is entirely a function of the combo of your personality and your kid's personality. Which means that your attitude has a large part in how it all goes, if not the actual interaction, then how much energy you lose from the interaction. (For me, I got angry sometimes, but mostly just felt defeated and so, so tired at all the conflict, so it was really about energy management.)
I felt like actual enforcement was not as important as having an explicit policy. So that when the child was violating the policy, I'd stop and verbalize the policy and then decide specifically if I was going to allow him to go against it or not. It didn't really change what happened in the moment, but gave me more of a sense of continuity instead of chaos.
I also stripped down to the essentials. If I didn't actually deeply care about something, I didn't care at all. In other words, I was choosing my battles, but at an extreme. There was a lot of humming of "Freedom's just another word for nothing left to lose" in my head during particularly testing times.
I tried to remember what I was good at. Most days it did not feel like parenting was one of the things I was good at, so I'd grab at whaever else I could: making pie crusts, knitting, listening to people's problems and helping them find a course of action, managing client expectations. In another few months or years this crappy strife-filled phase with your child is going to be over, but you have the rest of your life to be really good at driving to work by the back route no one else knows about and shaving 7 minutes off your commute. So celebrate your prowess.
Apparently other people have great success at turning things into a game. That wasn't my thing, but I know some of you will be able to talk about it in the comments.
Anyone else? How did you deal? How are you dealing now?