Oh Isla, I could write VOLUMES! My one and only son took forever to potty train. He was the most stubborn little stinker. I went through every self-help book, visits to the pediatrician..you name it. He had three different potties that I let him pick out (supposedly that was supposed to let him feel in control), I tried rewarding him with stickers, we had charts, Cheerios in the toilet for target practice...literally, you name it, I did it.
I'll PM you the gory details if you want them, but the finale was he potty trained himself in two days, only three months before his fourth birthday. Never had one accident after that. Apparently, once I gave up and planned on a life of diapers for him, he figured he'd beaten me and just decided to go on his own.