You've only had her a week? Well she will still just be settling in at the moment. All horses take some time to settle and particularly young ones you have to allow a while longer.
I have a giant 17.2hh baby here too and for the first 2 weeks she was, shall we say "lively". She's been here about 3 months now and I would say she has totally settled and is nothing like the skitty animal who came off the trailer back then.
Does she tie up outside? I personally wouldn't chose to groom and handle this horse inside a stable. You need to have space around you just incase she does head a whammer in your direction.
Buy yourself a walking stick and place a padded out glove on the end of it. Use this to touch and pat her back end. Use this to lift up her rear legs.
Getting her rugged will help you enormously!!! This will help you sooo much with her back end issues.
Sounds like she just needs familiarising to be honest. Don't give up just yet.....but DO be prepared to have someone knowledgeable on hand as this could be a bit more of a struggle than you think.
Don't forget you have the backing to do still......
Im going to take things very slowly. Im going to go down the yard and spend some time just being there, getting her to know who I am. I was thinking the next step would be maybe to take her into the menáge, where there is less fuss and screaming kids and maybe trying a bit of work with a halter, and if shes happy with this, maybe with the broom handle and glove.
just out of interest, her sire is Poetic Justic. Trying to find out a bit more on him. any info would be a great help.