hmmm - I'm certainly no expert on behaviour ... but perhaps she is grumpy as she may be coming into season?
Millie got a bit grumpy on the run up to her 1st season (which she had particularly early for a blue roan at 9months) but she was more 'depressed' than being dominant.
I'm afraid I haven't really any other advice to give, not knowing a geat deal about it .. but I'm sure one of the more knowledgable behavioural members will be along shortly and will post something more informative!
In the meantime I hope things improve for you with your little lady, don't give up just yet!
Kel
x
EDITED TO ADD - that perhaps you should try be more dominant than her? try not to 'cower' and 'bow' to her new demands and agression - make sure you re-inforce that you are the 'top-dog'. Try feeding her after you've eaten, ignoring her when you come home until she's quiet, and you are ready to say hello etc .. perhaps she just needs to be reminded that she doesn't and won't always get her own way ??