Gossipgirl I have completely lost it with my 2 year old several times in a spectacular fashion as well, when she really pushed my buttons. The guilt is excruciating but it is a very trying time I just try to resolve it afterwards and cuddle etc.
I've seen a good friend sit on her son upside down over chest, with her rear to his face, so she could keep him still enough to change his nappy whilst he writhed and screamed! Probably not the best way to do it but you are not alone!
My DD did go through a stage of being a nightmare to change at nappy time but she has now out grown it, phew!
The Rachel Waddilove, Toddler book has some nice advice in on ideas for tricky behaviour or the Charlie Taylor book call Divas and Dictators is good. Silly title though So maybe do some reading to see if you can form some more ideas for dealing with it all.
I found when I did not have a consistent plan of action for tricky behaviour I was more likely to loose it. Once you choose a method, either naughty step, go to room etc and do it consistently etc you feel more in control etc. They also get used to it as well in a weird way, although the tantrums don't go away they seem bearable in a bizarre way when you know how you're going to deal with it etc. Hmm that probably doesn't make sense, I'm having trouble trying to explain it.
I always try and warn my DD first before I need to do something she is usually problematic with and yes I may offer a treat but I find praise works better with her. Praise even for something small she's done. Asking her to do things to help me, she enjoys and I praise her for.
If she throws a paddy at home I tend to say "you lie there until you calm down, you have a nice rest and then come and find me when you feel better" when she does calm down and find me I say "that was a bit silly, mummy asked you not to do that, as you may get hurt" vary depending on situation and give her a cuddle.
Thing is, what works with one child won't work with another. You could go on a course at your local surestart centre, that maybe a thought?
But trust me, you are not alone on loosing it with your child and feeling terribly guilty after. The fact you feel guilty, shows how much you care and love them.
Sorry it's a bit of a ramble with lots of disjointed ideas and probably full of typos. Sorry