Sitting those big, powerful trots is tough!
It is very much dependent on the strength of your core muscles. You will see that the vast majority of Dressage riders spend a lot of time at the gym, and recommend pilates as an off horse exercise. Without strong core muscles you will forever struggle.
The core takes the brunt of the impact, holding your body upright and allowing your legs to hang passively. Your core will be burning after a short period of sitting trot if you've really got it working. This is what allows a Dressage rider to remain so quiet in the saddle. Then it is all about skill, feel, hours in the saddle, practice, practice, practice and a perfectly balanced seat.
You'll get it eventually, but you'll be surprised that it will probably 'click' best when you're not even thinking about doing it.