To be honest, her mental toughness hasn't improved that much.
She may not take it out on Gatorade bottles and balls anymore, but there is still plenty of negative emotion.
Maybe she just doesn't enjoy the rush of the big moments, of the high stakes matches as much as others do. I mean, take Sharapova. Without a serve, playing messy at best, shite at worst on her least favourite surface, against a tricky opponent and also against an increasingly hostile crowd at Lenglen, MP down. She has just been given another chance to prove herself, and she relishes it. She lives for that shit.
The stuff the punditry, the h8t0rz, even ourselves, say in here, about how she always fails to deliver in big matches against top opposition, particularly if things get tight, don't you think Nicole's camp and Nicole herself are aware of that as well?
Nicole hasn't shown much spirit in her most recent losses to higher ranked players this year. I wouldn't be surprised if she has lost faith in her own ability to compete with the elite - which is understandable, after so many heartbreaking losses and missed opportunities. All of that has taken its toll, and would probably have shattered the confidence of a titan, let alone of a 19 y.o. under pressure to deliver.
If you've read my ridiculously verbose writeups on GM, you'll know I'm afraid most of the problems with Nicole's game are unsolvable. That wouldn't be disastrous in itself though (recent experience shows players shooting up the rankings on account more of lucky breaks than actual improvements in their games), if Nicole hadn't shown to be somewhat injury prone (one of the reaons: too much wrist on all of her strokes). She can't schedule like Chak/Sveta/the Serbians/Danka (i.e. whore every tournament in sight and keep her fingers crossed in the hope of landing at a section of the draw without Henin, Serena, Sharapova or somebody else who happens to own them). She has to play a limited schedule, entering relatively few events, usually those with stacked fields. This means that, to get back to the top 10, she'll have to beat top players, something she has so far failed to do on a consistent basis. So right now she seems to be at a dead end.
But don't get me wrong; I think her team's scheduling is brave and commendable; you can only achieve actual improvements by playing and beating the best, not by was bulking up one's ranking and titles tally out of tournaments with depleted fields and out of taking advantages of openings on draws.
This being said, and on a more personal note, my attitude towards Nicole has always been different from the one I have towards some of my other faves. I often got seriously pissed off at some of Lena D.'s losses since I thought (and still think) that, had she managed to build a proper game, including a halfway decent serve, around her unsurpassed athleticism and ball striking skills, she could have been another Clijsters. With Nicole, I never get angry or feel that disappointed, no matter how awful the loss (exception: loss to Krajicek at Charleston) since I've always found her game occasionally spectacular and always wildly entertaining, but ultimately ineffective. With Chak it's a bit the same: I'm amazed that she wins matches at all, and I'm always very protective towards her, going "Ooooh, poor little thing, always up against cavewomen twice her size!", even if she's playing Camille Pin.