I have to say Coss I am consistently impressed with your responses to schooling related posts.
Your right, she does struggle to go in a straight line and did use to forge but this is improving as her schooling gets better - she had me frustrated for months at how difficult it could be to trot down the centre line!!
She also finds collection difficult but is getting there, although she does tend to get tense and her tail goes round like a windmill, plus she will buck if you ask for too much.
As you said, shoulder in has been really useful but if you have any other suggestions I'd love to hear them. Honestly, I wish you could give me lessons!!!