Sit tall, sit deep, sit relaxed, keep legs off - infact to make sitting trot comfortable you need to keep thinking "legs apart" and sit on your seatbones. When I mean legs apart I do not mean literally, though I do use this exercise when teaching people on the lunge and a pre-requisite of lengthening the legs and getting them back, and also for finding the seat bones.
The half halt might help, just lift your outside hand slightly and then put it down. And resist the temptation to continually pull to get the horse to slow down, rather block with your back - making it less flexible than you would say, if you were swinging your back to walk.
I would resist cantering altogether for a few days. Without seeing what she is doing, her head carriage, her demeanor etc, I am not able to say just what is going wrong. It could be that she is tense or has a sore spot in her back (very common) and she might need some chiropractic sessions. If she does canter off when you try to sit the trot, just quietly bring her back. No big deal, just bring her back and tell her "good girl" when she comes back to trot.