Not sure that this is going to be of much help, but my understanding of it, when they behave in this manner, is they're doing it for attention. The toddler is the centre of attention as mum does her level best to keep toddler happy. I know toddlers are a little more complicated than that, but that's my initial guess .

Not that I'd suggest you ignore your DD (not at all!), but would it be worthing letting her "sort" her small problems out on her own - frustration or not - and save your help for when she really needs it? As for wanting juice one minute and the next, I can only say what I'd do if it was my DS1 ... which would be to not give him any juice if "yes'd, no'd etc". Then if he threw a tantie, I'd just act completely normally and not fuss over him till he stops.

My DS has only just turned two and while he hasn't hit the "yes ... no" stage, he will sometimes throw a wobbly when he doesn't get his way, or when he wants something he can't have. I find the less fuss I make of him when he's being precious, the quicker he gets over himself. I find distraction to be a good method to avert a meltdown too.

Parenting a toddler certainly is a sharp learning curve! Good luck.