it's pretty hard to see. A zoom in would have made it easier to see, and it's hard to see you leg, being dark pants on a dark saddle.
Anyway, all my whining aside, your posting does not look well timed with the push of the horse's hind. Even at the very beginning, you actually take your first rise in posting before the hrose has even started trotting, so you are posting from your leg, rather than from the push of the horse.
The effect is that you post off of your knee, as a pivot point, with you leg seeming to be not carrying the weight it should . Any time you have a pivot point at the knee, the body weight does not travel down into the lower leg. It makes you less stable and prone to lurch forward if the horse stops suddenly.
I couldn't see very well, but I think your stirrups might need to come up one hole.
I know that her trot is probably rather hard to sit, but you might do some sitting trot work, so that you start to feel the push better and allow it to move you in the posting trot, rather than trying to create the posting on your own.
Also, the contact you have on her mouth is a bit "dead". I mean, you are not really achieving anything from it. She is not flexing at all to the rein.
I would probably give her more rein for awhile to just work on getting her more forward (which will make posting her easier), then, work on getting her accepting of contact and giving in her jaw and poll, but do that at the walk.