D.D.
First off, you sound like a terrific mom, and like you are doing all the right things. You are not alone in this.... we all know how you feel.
My boy just finished training, at 3 years, 2 months. I tried the charts, the stickers, the rewards, etc. Nothing helped. I finally decided to back off and just let him do it when he was ready, because he had shown interest, and could do it all from a physical standpoint. Then I started to do things differently. The first thing I started doing was to move all his diaper changing stuff into the bathroom. We started talking every day about the fact that the bathroom is where poop and pee belong, and he started flushing the solids in the toilet. We did the entire diaper change standing up, right next to the potty. It just seemed to me that if he was doing all his diaper-related stuff in the bathroom he might get the idea. I think it helped, because he started doing well at day care (school) but not at home. School was let out in late May and in order for him to move up into the "big boy class" he HAD to be using the potty all the time (well, except for accidents). So, on Wednesday before he was to move up the following Monday, I sat down with him told him the facts. Use the potty or stay behind with the babies. It worked. He flat out started using the potty full time that day.
Anyway, I think that if the push about school had not come up, he might still be using diapers. I don't advocate pushing a child to do anything, and if my boy hadn't gotten on the potty train that day, I certainly wouldn't have pushed him.