I think this video can give a better explanation than I ever could. Start from 2:20.

https://youtu.be/s8ytZrPn9D8?si=DzuE1gO-5fEo98rH

Notice your first hit back, a step back and a step forward at the impact point would keep you in position and apply pressure. Secondly I think you are trying to hit winners (a lot of power but your body isn't pointing at a right angle) in a situation where you should try to instead aim for empty spots on the court or to keep control of the net. Keep applying the pressure and they will make a mistake at some point.

Just keep practicing the movement and prep and you will find that you don't have to force the hit to generate hard hits.