There's following the horse, and then there's blatant pumping.
Your elbows need to be glued to your sides and your hand resting on the saddle pad, you're not pushing a shopping cart. In the rising trot try to rise smaller and not hump the air every trot stride. This is pushing her out of balance, onto the forehand and causing that slow flat trot. The less you can move, the more the horse is able to come up out and in front of your leg.
I disagree that your elbows need to be bent more. They need to be at whatever angle allows your upper arms to be glued to your body and your hand to be glued to your saddle pad or her withers. The elbow will then open and close to absorb or "follow" the horse's movement. That's the only following that we want. As far as the position of the hand, don't focus on thumbs up as clearly that doesn't work. Think about flexing the flexor carpi ulnaris muscle (google it) which pulls your pinky finger closer to your elbow. You'll notice that doing this with the thumbs pointed in it is almost painful, but with thumbs up is easy. From there think "pinkies UP" to keep rotating the hands outward.
I know there are people on here that disagree with that philosophy for arms and hands. However, soon, soon I will get a video up of a horse with a bad walk, who has been ridden with a stable hand since he has had contact (for only a few months) and the improvement in his walk and self carriage is immense. The horse has gone from w/t/c in a wild fashion to schooling at first level and playing with some second level in a very short time with a stable contact and increased gait quality.
Anyways. Your basic position is very good and I have only a few nitpicky issues with it (hand/arm position, as covered) but it is the functional equitation here that is causing your issues. In pictures or frames, you look good, but on video everything is very loud and big. Remember horses can feel a fly on their back and flick it off!! Try to be more and more quiet on her back and as much as I hate the word, be still or statuesque. Think of this until your habit is broken or until you are too passive. Riding habits fluctuate in a pendulum like fashion. You need to become too still before you can safely revert back to the "center" on the stillness scale. At this point I think you will find the horse more willing to go forward. And let her carry you!! Be dead weight sometimes! She will enjoy having a job and not being interfered with. Also work on the response to the leg. Be completely still and only apply leg. If there's no response then follow the normal progression of aids until you have a response. At some point she will realize that you are no longer doing all the work for her and that she needs to "buck up"!
Good luck! I think your position is very good, now you need to enter the realm of functional equitation. Remember that the horse knows better than you how to walk, trot and canter. You have to let her do the gaits and just be a passenger.
They say money doesn't buy happiness -- well happiness doesn't buy horses!