I remember the dread of compulsory sitting trot when I had a horse ready to go Intermediate but was still really struggling with my seat in sitting. Of course the harder I tried, the worse it got. This is the tip which helped me the most, in case anyone else out there is struggling to relax and let sitting trot just happen.
Sit a few extra beats at a time, so something like up down down, up down down, up down down down, up down, etc, gradually decreasing the number of rises. I found this really helpful as I could do a tiny rise or two when I started to struggle, rather than tensing up and gripping, so the horse’s back stayed soft, increasing the likelihood of me being able to sit for longer next time!
Eventually it became much easier to just relax and sit all the time. Having the trot a little underpowered at first helps too… and of course if the back is accepting the rider and swinging then it makes sitting much easier.