Something else to consider may be how you walk. Do you walk in a straight line or tend to "spoon out" (as my trainer calls it)? I usually "spoon out" when I walk, so I was constantly cutting off my dogs when we are walking/heeling, so they would get behind or ahead of me, and in turn, tend to not sit in the correct position. Does she sit on her own, or do you give her a "tug" to remind her to sit? If you have to remind her, are you pulling back or pulling up? Pulling back could make her sit back on her heels, essentially sitting back instead of sitting "down". When you are asking her to "halt," are your shoulders parallel? If they aren't, then she could be taking the signal from you and sitting back of heel position. Like Red said, heeling and sit at heel are the basis of most everything advanced you'll ever do, so make sure you take your time! I'm in the same boat, I really want to rush because it's fun (and I'm a little overcompetitive!) but my trainers like to remind me of my errors and things that my dogs do that aren't automatic or exactly right so I have to slow down and make sure that what I am doing is telling my dogs the right thing to do.
Have fun!!