There can be many reasons that a horse pulls on the reins, they can be leaning on your hand, they can try to bolt and they can sometimes just be moving forwards.
My Gelding gets strong in the canter, not in an impolite way it’s just he moves forward and into my hand so I feel some weight through the reins.
If your horse is leaning a lot or trying to bolt, I would recommend slowing the pace and either circling using some flexion and contri-bend to get the horses neck moving and also incorporating lots of changes of direction - straight lines and long sides can be where horses start to fall flatter and lean more as they don’t actually have to do anything except forward.
I hope this helps and kinda makes sense.
you should try lunging before rides or if you have a free arena let her run loose once a week. if that doesn't work, try letting her canter or trot putting your reins completely forward and let her go until she gets tired. also, you could try squeezing your hands and wiggiling your reins when you ride to give her something to focus on