P.M.
Based on what you write, I think the only thing you are doing wrong is having tried so hard to train her for the past year. That's an entirely different dynamic than waiting to train her until she's ready. She has a whole year of frustration (yours and hers) and failure to deal with now, and so she may understandably feel quite a bit of resistance that may not have needed to be there.
How about telling her that it's up to her, and then putting her back in diapers. She may need a week or a month to be sure you mean it. Then she'll be able to start deciding for herself if she's ready. She may not be, but probably will be sometime in the next several months. Readiness must be both physical and emotional. You can't make her go if she won't, or can't.
Yep, it could still be a long time to wait, but it will be so much less stressful for both of you if you give her the space she needs. For most kids, when they are truly ready, they are excited about it and essentially train themselves. But your daughter has an ego to defend, and it sounds like you've put her in a position where she believes she needs to defend it.