Whether she's lost her grown-up privileges as part of a temporary punishment or a permanent change in her lifestyle, it's normal for your girlfriend to sulk a bit. She might complain that wearing baby clothes makes her look ridiculous, or that wetting and messing her pants is disgusting. Tell her that's the point.
It's fine to let her know you think she looks cute as a button in her silly little frocks, and to reassure her that you have no problem changing her adult diapers, but don't let her forget how absurd it is for a woman her age to dress like a toddler and go to the bathroom in her own pants, because it's the humiliation that will break down her ego and turn her into a sweet little girl who knows her place.
There's no need to mock her too cruelly (and any teasing that results in tears should be immediately followed up by a lot of cuddles and kisses), but I do recommend making fun of her on a regular basis. Ask her what her friends would say if they could see her with her legs up on the changing table. Comment loudly on the state of her nappies whenever you check them. Call her infantilizing names like "potty pants" and "princess puddles". If she's constantly pouting, that's a sure sign you're on the right track!
