Is your coach worried about you 'getting her head in' or is it just that you've seen pictures and think that you need to have the horse going like that?
Totally agree with Anebel, you say you've been riding for 6 months. You are no where near ready to start learning collection. You need to (again, to re-enforce anebel's comment) find someone who will put you on the lunge with no stirrups and reins, and learn how to find your own centre of balance, to the point where you do not rely on your legs or hands to balance in the saddle.
From there, your seat will begin to influence the horse, take your reins and stirrups away and have the coach teach you about driving the horse into the bridle. Forward is ALWAYS the key. You want the hind hooves to land in the tracks the front hooves have made. Once you're horse is tracking up, you know your on the right track (no pun intended).
It's very common for beginner riders to have the horse plodding along, dragging its feet and the horse will have its head 'down' giving the rider the impression that the horse is 'on the bit' or 'collected'. This is not the case and a good coach will explain this to you ;)