Thanks guys, I really appreciate the supportive comments.
RosieB had asked on the other thread what I thought was the most helpful tip for getting relaxation - I think probably (and this is not surprising or rocket science!) simply sitting up and controlling the posting was the most effective, both because it got her back a bit, and getting her back even a little made me feel more confident that improvement was possible. Also the reminder that it is ok if it's not perfect right away, or if it gets a little ugly, to not get caught in that and to have confidence that it will get better.
I've said this before but it's VERY clear to me after this experience that I go into a very negative, pessimistic headspace very quickly - the more pressure I'm under, the faster I give into despair. If it isn't going well right away I feel like I'm never going to be able to make it better, and will probably only make it worse. But seeing this video makes me want to grab past me and be like - this is fine!! Even the "bad" isn't THAT bad, just sit back an inch, slow your post and have a little faith in your saintly, hardworking, amazing pony!
I've had tests like this, where I was so convinced the ride was total trash and that we should just excuse ourselves, and then I get my test and the judge thought we were doing great, or I see a video and I don't even recognize myself, my mental picture of what was happening had been so distorted!
So instead of focusing so hard on my position or exercises, I'm going to try to focus on my mental attitude, and work to strengthen it the way I work to strengthen my core.
On controlling the posting - I find this idea very difficult, I'm going to practice this more - I'm so used to following the horse's rhythm, posting my own regardless of what's happening under me feels so weird and wrong, even though I know it's the right thing to do, and it certainly isn't the first time I've heard it.
