In a battle of strength, the horse is always going to come out on top.
Riders often react to a strong horse by taken an even stronger contact.
This often allows the horse to just simply plough into hand and get faster. You need to have the opposite reaction, so every time your horse gets stronger, you ask for a downward transition and then give (ie soften, or allow forwards with the hand) the contact. This acts as a reward for coming back to you, while the transitions also help engage the hind quarters.
If your horse has a tendency to be heavy in the hand, the following quick exercises might help.
Ride a three-loop serpentine in trot, asking for a transition to walk before you cross each centre line. Stay in walk for five to seven steps then return to working trot. The turns will encourage your horse to listen.
Leg yield from the three–quarter line to the track. Every time your horse gets faster or leans, half- halt, channel him straight with both legs and hands, and then do a few more.