hills, hills hills, then when you're bored of hills, MORE hills :p
lunging *correctly* in an outline with hocks right under them can help, as can lateral work whilst schooling :) that should also help with the leaving them behind etc- a horse used to working correctly usually knows where his...
horses can come back after much longer periods of time off, i wouldnt worry about it :)
youll probably find even if u pop home and drag her out the field every now and again, she'll be absolutely fine!! (god knows, i did with mine!)
schooling etc will take a little longer to come back, but...