Hi Hannah and welcome to the forum. Do you have any pictures of your horse you could post as that is always helpful? What do you want to do with her - pleasure ride, compete, etc.? Quite frankly, if it was me (as I'm mostly a trail rider) I wouldn't worry too much about the headset as long as she is going along comfortably and quietly. On the other hand, if you are interested in showing, the cards may be stacked against you if she is indeed downhill, perhaps short necked and possibly straight in the shoulder - you'll have to work that much harder to get her collected properly and it may never look as elegant as the horse with more balanced conformation. I don't hold with doing headsets as such (it only fools the unenlightened
); as JustDressageIt said - get the body working the way it should and the head and neck fall into place to the best of their ability.