I just watched your video and looked at the pics in your album.If you are wanting to do western pleasure you are defiantly going to have to work on headset and building more muscle all around.Maybe the western pleasure riders can speak up and give their opinion.
Trotting is a great way to build up muscle,Up and down hills,working circles, are a few that I can think of right now.Come up with a workout schedule,on the days you are unable to ride,longing is a good way to keep them from getting sore.
Looking at the last pic,she appears downhill which is making her whole body look outa proportion.Its going to take alot of work getting her into shape,and training her for the correct "jog,canter"...
This link has some info on requirments for WP Western pleasure - Wikipedia, the free encyclopedia
-- Horses - Western Pleasure Riding and Showing on Equiworld - Equestrian Information on the internet
--Riding Horses in Western Pleasure