The photograph won't open for me.
But going on what you've said, I would just be feeding a balanced yearling diet. Plain oats fed with ample meadow hay and a biscuit of lucerne each day balances the phos/calcium ratio and most youngster thrive on it.
As a yearling, of course she is going to be gangly. Don't even worry about her looks at the moment. My yearling is a very well bred hanoverian gelding, and right now he closely resembles a small camel. Scrawny, gangly long legs, bum high, big head, little neck..... But he's a good weight and has a brilliant coat, so I know his diet is working well for him and his growth rate.
He gets fed - and I am planning on doing some in hand breed shows with him over the next few months - 1/2 biscuit lucerne/day, full turn out in a very good paddock, 500g wheaten chaff, 400g Pryde's BioMare, seaweed meal, sunflower oil, 1 tsp MSM and 20g Kohnke's Cell Grow (young horse supplement).
This is him 2 days ago, at nearly 16 months. He looks fat in this photo, but he does have light ribs showing as a youngster should.