Not sure if I have any advice for you but can certainly sympathise. DD was trained at around 2 3/4 and took a few weeks to really get the hang of it - ie she understood what was expected, was physically capable of doing, communicated well enough to ask etc. Nevertheless she continued having regular accidents for months. It drove me completely up the wall. I would get so angry.
About 9 months after she was out of nappies and still having accidents I felt I had to do something about it. By this point accidents were only ever happening at home, not at the childminder or friend's houses - so I knew it was more of a power struggle than anything. She's always been stubborn.
I asked lots of experienced people for advice and they all said just ignore it. If she has an accident just clean it up without comment, give her some dry clothes and move on. Don't say a word. (Up until then alhtough I never told her off for wetting herself I would always let her see that I was upset with her). So we tried it. For two weeks. And do you know what? It got worse! She started having 2-3 accidents a day instead of 2-3 a week.
So despite all the advice DH and I agreed to try a different strategy. We made it clear to her that accidents were unacceptable and wouldnt' be tolerated. That day we went to a friends house and she was promised she could stay for dinner and a bath there - a real treat! She was very excited. Throughout the afternoon the kids played and we occassionaly asked dd if she need to the toilet. She repeatedly refused even after several drinks. So eventually of course, she wet herself. Well, we changed her pants, took her home and put her straight to bed. She was distraught. And that was the end of the problem. Period.
She's very rarely had an accident since then and usually only now when she's not feeling well.
I've just started toilet training ds, now 2 1/2 but I'm trying not to let the stress and PTSD from that experience affect how I'm dealing with him. He's a very different child and I'm hoping it will be a very diferent experience.
So my sympathies because I can imagine how you are feeling. Good luck.