I recently tried this exercise and it really worked, while trotting or cantering every stride do a slight tug on one side of your reins then alternate tugging on each side of the reins this really helped get my horses head rounded
You ideally need a trainer to help you with this issue. And make sure your trainer teaches/rides the CLASSICAL way. Not pulling the horses head into frame, but pushing the horse forward into a soft and buttery contact. It gives me butterflies in my stomach just thinking of it 🥰.
You need to have a soft and elastic yet consistent outside hand. Think of it as your holding your horse's hand saying it's gonna be OK.
Then, push your horse forward with your seat into your hands.
You want to think of it as your horse is a spring, when you push forward from behind, your hands don't pull back but the horse's bum comes closer and underneath him, therefore bending the spring into a nice and soft, supple and elastic contact and from. If you pull back, your horse is going nowhere. You want him to go somewhere, so ride him forward and then just hold him a bit and tell him that his bum must move forward but his front end must stay with you. Again NO PULLING.
It is a difficult concept and it takes many professionals years to master. But this should steer you onto the right track.
Another way of thinking about it is if you pushed the accelerator in your car but held the brakes down just a bit, you would feel all that power but you are not going fast.