Oh.... the joys of a toddler asserting himself !! :~(
If you've tried all the suggestions no wonder you're frustrated!
Zgma, it sounds like it's a touch of the 'Terrible Two's'! And I don't think it is a power struggle.
All toddlers, as they grow up, start getting 'independant' and want to do things for themselves, even if the end result isn't quite what we parents would prefer!! Sometimes it's best to back off and let them get on with it, unless of course what they are doing is dangerous.
Often the best way to deal with these 'tantrums' is just to ignore them, as the more you try to control the situation the worse it gets and you get wound up. By ignoring them, the child gradually learns that it gets them nowhere and gives up. These are just phases as he tries to assert his independance, and he will grow out of them. The less 'fuss' you make the less he will. Give it a try and see what happens.
My sister had loads of problems with her daughter, laying on the floor of the supermarket screaming and kicking, not eating, refusing to wear anything except pink trousers and t shirts etc. All my sister did was say 'fine', and carry on with what she was doing. One day my sister even left her daughter (warning her 1st) on the supermarket floor and walked down the aisle....result ....instant silence and a docile daughter!!! Now she is 9 and well behaved...mostly!!