umpadink - have you been to the doctor to check for a uti? my friend's daughter had v similar experience and it was an infection.... or does your gut tell you something different? (presumably boys can get utis too, I am not medical as you can tell....)
and also you are far from the worst mum in the world, as we have ALL been there on the losing it front and got the t-shirt.... it's tough and infuriating.
if you can rule out something physical it sounds like the Freudian thing that many of the other mums on here are experiencing. Can but doesn't want to / can't be bothered.
BUT
My theory on this having done the amazing story thing with my dd is that consciously or, more probably, unconsciously our offspring are using this as a form of CONTROL over their bodies and situations that may have got out of hand for them. So rather than "oh dear, mummy is cross I mustn't do that again" it's more a case of "arghghgh! something is wrong, mummy is cross, I've lost my place of safety, will mummy be cross if I do the same thing again? arghgh she is, oh no, what if I do it again" - I don't know if that makes sense to anyone but I think a lot of my DDs behaviour was coming out of a reasoning system like this. It was as if on some level she was testing me, but she was also genuinely mortified if I was angry with her and it always seemed to make the weeing/pooing in inappropriate places much worse.
As the unconscious is the language of dreams, stories and fairytales the way to get into their lovely little heads is to use stories and create a place of safety for them in relation to their toilet habits and also practice the EXTREME art of zen mothering and not react at all to the puddles and/or parcels in the small underwear (arghghghghgh!!!!) .........other than to clear the blimmin stuff up BUT to instead work on creating that place of safety for them again in their unconscious.
Maybe a story of the sad wee who wanted to go down the toilet and got lost in the soft play instead or something could help.... (see the poo goes to pooland threads above) I think we have to let them know that they are indeed in control of poos and wees but steer them in the direction of 'doing the right thing' - making the poos and wees happy by putting them in the right place. My DD is so keen on poos now I have a job to stop her straining on the loo or potty at every available opportunity - and she always wants them to be happy as we flush them away to their friends in poo land! I know it sounds BIZARRE but it's powerful old stuff and the ONLY thing that's worked in 18 months.