NSHorses tend to have a very upright neck carriage by build. It can be hard for them to bring their head down and not brace against the bit, and having them move around "giraffing" is hard on both horse and rider. Such a horse makes it hard for the rider to become stable enought to then give the hrose a quiet enough hand to get it to trust and come to the bit. IT's kind of a vicious circle.
That's why I would consider doing what Puck suggested, the relaxed circle with encouragement to come to the outide rein, reach for the bit and carry it downward and forward , be done at the walk for some time , at first. That way your seat can be stable , allowing you to be effective.
Once you speed up, things get that much harder.
I've also ridden a friends Morgan mare who is built very similarly and she is tough to get to relax and accept the bit. Lots and lots of half halts are needed for her to relax and flex at the poll.