I would first like to say that I think you are a nice rider with quiet hands. Your legs could be longer but I think that is the saddle positioning them there. I really like this horse.
She is nice and relaxed and from what I can see fairly rhythmical.
When you warm up your horse, you should refer to the Dressage Training Pyramid. This is :
The idea of this pyramid is that you can not accomplish #2 without #1... Therefore you can't accomplish #3 without #2 and #1. Basically, you can't get to the next level of the pyramid without having the previous in tact.
Duke is my draft cross who is doing acceptionally well in dressage. Very naturally built front end and hind end for the sport. Nice loose mucling throughout. However, his main fault is he gets a bit "worked up" sometimes. EVERY TIME I ride a horse I have this pyramid in the back of my head. Because he is a bit more advanced in his training, I don't really need to spend so much time on each individual step because it's becoming more instilled in him as a whole during every ride. When I gather my rains he comes directly into frame and on the bit lightly. He usually needs help with his Rhythm (or tempo) at the beginning of rides and sometimes throughout. What I am saying, is your main goal on a horse is to accomplish these things. Think of the pyramid as layers to your horses training:
The first layer you need in a ride is Rhythm. The rhythm that has energy and tempo. Every stride is the same! This tempo needs to be not too fast where it is running, however, it does need to be in front of your leg... NO EXCEPTIONS.. forward is the ROOT to many training problems. A good way to establish a rhythm is just riding large at a very consistent pace. If she rushes, half halt. If she ignores the half halt, walk. And then do it again until she learns where her correct tempo is.
The second layer you need to your ride is relaxation/suppleness. Relaxation comes easier to some horses than others.. The relaxation must be within the rythm that you have established during the first layer. A good way to achieve the relaxation is doing a lot of 20 m, serpentines (if she can balance herself through them, otherwise it would do the opposite of relaxing), continuous changes of rein.. Another way to put relaxation is suppleness. I want you to imagine that your horses is stepping every single stride over her back, swinging as she does it, and into your soft hands. Your arms and elbows must be soft and non-restrictive to do this. DO NOT BY ANY MEANS wiggle your horse into frame. This does absolutely nothing for their top lines or hind end. When you begin to do that, they are posing, not moving from their hind ends and actually hollowing thier backs. This will come back to haunt you in the uper levels. It took me a good 2 months to get Duke to want to swing in a nice stretch from his hind end, over his back, to the contact. Suppleness means you can move a horses neck laterally and longitudinally. You will know when your horse has successfully accomplished an elastic, "chewy," forward suppleness. It is a wonderful feeling. Honestly, to me this is all an imagination thing. You need to visualize it and how you are going to mold the horse witout wiggling it down. This is not to say you can't use flexions or squeezing of the fingers. Just make sure that those aids accompany your leg and seat to get it accomplished without gimiking.
The third level of the pyramid is connection. This means the acceptance of the bit through the acceptance of the aids. The horse will always be on the bit and into contact at this point. If she goes above where you have placed her frame, yes.. you can use a little corrective suppling aid with your hand however push that hind end through as well! I have found through Duke's training that when he comes up, it is directly related to the loss of pushing power! Which explains why it mostly happens in corners (also the loss of balance).. This is a strength and conditioning issue that will take time to address.
I will not discuss the rest of the pyramid as that is not your question. Going through these 3 steps is a warm up! I don't believe in allowing a horse to run around with their heads straight in the air with no rhythm to "warm up their muscles." A horse should learn early on in their training that NO MATTER WHAT, they stay supple and over the back. It will take time, time, and more time for a horse to have this imprinted in their brains. Muscles are made to retract, we must teach that muscle memory to do the opposite. Muscle memory takes a long time to retrain especailly on an animal. In your warm up, think of these three things continuously. Keep going through a check list. Is my horse rythmical? If yes that move on, if not work on it. If yes: Is my horse supple? If yes than move on, if not work on it. If yes: Is my horse accepting the contact through my seat and leg aids? If yes. You've had a successful warm up. Keep doing this every ride until your horse builds strength.
I attached a link to a video I think you will find helpful. You need to see how a forward, supple, into the contact horse looks! :) hope this helps. PM me if you have any questions and sorry if it was confusing. (And I don't like how curled this horse is at the stretch, but its the only video I could find the directly discussed it) Cesar Parra on the Stages of the Training Pyramid - YouTube