Thankyou for the cute video. Lena is really a nice looking horse and goes pretty nicely if you ask me. The thing I immediately notice is not her but YOU. You are leaning so far forward and have your legs crooked up and I am not sure, but it almost looks like your heels are gripping into her sides. Mostly, your leanig forward (and I saw this throughout the video) puts you horse off balance. She struggles to keep you balanced because your position is weak. Evidence by when she came to quick stop, as you had asked, you fell forward.
I did see her can't her hind end in a little on the canter, but as for her being able to engage her hind end, it's realy hard for her because she is heavy on her forehand , in part due to her trying to keep you balanced when you are leaning too far forward.
She is really a cute and responsive mare. I really like her and you can get a whole lot more out of her when you get the most out of yourself, and you have a lot of potential as a rider.