In MY case, it sometimes helps to shove my feet a bit forward. If my legs angle forward, it is easier to keep my knees loose. That may just be my shape & tight hips. Some people brace against the stirrups with their feet forward, but I don't. The wider the horse, the more difficult I find it to have loose knees AND heels below my hip.
I also find it easier to sit the trot after riding for 20 minutes, so maybe you could post early on and then try sitting the trot for short times. I'm 54, and it just takes my body a while to loosen up.
I'll add that my horses seem to prefer it when I post, so I post most of the time. Sitting the trot, for me, is about working on my body's flexibility...so I only do it for short periods of time.