She's not regressing- she was never developmentally ready for this in the first place. She's not sleeping, and neither are you. Put her in a diaper or pull up, tell her that her sleep is much more important, and forget about it. If she's still wet at age 6, see a pediatric urologist - but at this age, she is completely normal and the 21 month thing was really a fluke. The bladder is not yet ready to send a "full" signal to the brain, which isn't ready to wake her up. This is going to stress you all out if you try to push it - it's not a question of "training" at this point - it's purely physiological. For many kids, if they are pushed, it makes it much worse in the long run.