It becomes a battle of wills at that age. My daughter went through the same problem. I read in Parent magazine that when a toddle thinks they master something, they move on without a thought back on it. With my daughter, it was summer, so it was a little easier to do this. I let her run around naked. Without the diaper or training pants on her. If she peed on te floor, I made her help me clean it up. I involved her in the process, which gave her some control of the situation. I had a drawer where I kept her diapers at the time, and I replaced them with panties. She would go to get me a diaper, and there weren't any. She would get a little angry with me, but I told her they were all gone. Now, I still put a trainer on her at night, which seems silly, but after awhile she stopped wetting it at night, and I moved to panties all the time.
If she had an accident in her panties, I made her stand in front of the sink and rinse them out. I also made her help me with cleaning the area she had messed up. Changing her clothes, bed sheets, etc. I guess it sounds a little tough, but at that age they can grasp the concept of consequence. Before too long, she realized that having to stop playing to help clean up her mess took longer than stopping to go to the potty.
It was a process, and it didn't happen overnight. We had accidents, and I never berated her for them. I was very patient, which was hard for me at the time, but I took the time with her to help her understand. But it did happened. It's all in the approach, I guess. Every child is different.
With my son, he was three and I had just had his sister. I was chnging her siaper one day when he walked up to me to tal me he pooped and needed to chnage his diaper. I was so tired, and coudn't imagine doing this for the next few months. SO I looked at him, and I told him that this wasn't going to happen again. He was old enough to understand when he needed to go to the bathroom. I also took away his diaper drawer, filled it with underwear, and told him from now on he wasn't wearing a diaper again, even at night. I went through the same process with him. He hated cleaning up his mess, and it only took a few mistakes, until he made less and less of them.
I hope this helps you. Good Luck! email me at ____@____.com if you need to vent. And you will.