A. ~
I was actually glad to read your post b/c I know I'm not alone. I have a son who will be 4 in August. He's still not potty trained! He knows how to do it (My birthday was in October & he used the potty ALL DAY & said, "Happy Birthday, Mama!" each time!) We tried Cheerios, Sticker Charts, A Penny For a Small Gumball Machine we kept on the potty, Food Coloring, & Bubbles ... nothing worked. We tried switching to just underwear for TWO WEEKS ... he just went in his underwear & CHANGED HIMSELF! It was frustrating. Both my older daughter & older son were trained easily by age 2years.
We decided it was a defiance issue & DROPPED IT. In the last couple of days, our son's started putting on "Big Boy Underwear" himself & going potty - both pee & poop - a/b 90% of the time. We just hug him & tell him we're proud, then drop it.
I KNOW it's hard to hear that you just have to "let go & give up," but if your child's like mine, this may be more a/b control.
I wish you the BEST of luck & a LOT of patience. We're seeing the light @ the end of the tunnel ... hopefully you will, too, soon!
[update: we told our son that we were "Stopping the potty training" ... we showed him the drawer where his Big Boy Underwear was kept & told him it was "up to him" to use them when he wanted ... w/in ONE week, he'd put on a pair of underwear & he has NOT worn pull-ups since ... totally trained HIMSELF in a week!]