Cactoosh it sounds like you're having a rough time. We all do things we're not proud of from time to time, because our tethers are not infinite and we're human. That doesn't make you a bad parent, and you have not failed. Consistency is definitely important, but in order to be consistent you have to totally believe in what you're doing. Advice from others can be useful (although not always ), but personally I've found I have to cherry pick from my gut feelings, books, other parents, whatever, and then bore DP to death using him as a sounding board while I work out what I really want to do (discounting tying DD1 to the bed to give everyone a break, even if that IS what I really want to do).
This isn't advice, as I don't feel qualified to give that, but it is a sharing of what has worked for us, if that doesn't sound too poncey.
I try to look at it from DDs point of view. I know that sounds all huggy, but it has several times made me sit back and think, actually, have I asked her what the problem is? Have I set out, when we're all calm and not shouting/hitting/whatever, what my problem is with the behaviour? Have I asked her for her ideas for solving it? (good for things like toy-sharing issues, tantrums over how/where to clean teeth etc) Sometimes DD has brilliant ideas for solving problems that I would never have thought of. Equally she has mad and totally unworkable ones but that's part of it too, to make sure she's listened to even if I can't actually make it happen.
So, on that theme, wild over empathising. 'I can see you're REALLY angry about that. How about beating the shit out of that sofa instead of your sister?' Obviously I paraphrase, and I'm aware just how deeply American-therapist it all sounds and yes, I felt like a total fraud doing it for the first time and no, it doesn't work particularly well any more but the first few times she gave that sofa hell and ended up giggling, and then she kind of got over the angry ranting phase.
My own behaviour - I'm very aware that we have our most rubbish days when I'm tired, grumpy, stressed etc. DD is a nightmare when I am too. So I have to force myself to be the grown-up, to be kind, to remind myself that kids learn by example. And yes I've generally lost the plot and have thrown something by the time I get a grip on myself again, but at least it's a way back from the brink. Oh, and making sure that one, three, five explosions don't = a write-off day. I try to put each meltdown (hers, not mine) behind me and start afresh each time. Otherwise I really would have killed her by now.
Btw, it's all very well the HV talking about positive parenting, but did she give you any tools to help do it? And you can't ignore dangerous behaviour - violence (against people or property) is non-negotiable however positive you're being. When people talk about not 'rewarding' bad behaviour I believe they mean not giving lots of attention as a result of it, not completely ignoring it, you've still got to intervene if someone is getting hurt. DD has only really taken her frustration out physically on me or DP, in which case I've manhandled her to a safeish place, told her (calmly-ish) that I'm putting her on the bed so she won't hurt herself, that I can't let her hurt anyone else, including me, and I'll be downstairs when she wants to talk to me.
Sorry that's really rambley cos I know DD2 is waking up - got to go. Good luck.