Sitting on a "baby"'s back is going to do no harm, however bouncing off it every stride will.
You're stiff, completely braced against her movement, your leg is swinging and your hand is cemented in one place.
If I was your coach you would be in a dressage saddle with no stirrups or reins on a lunge line learning how to be soft. Our horse's are mirrors of us, so when the rider is so stiff and unbalanced in the tack, the horse is going to be too. It is because of this I can't really say too much about the mare other than she seems to have a good work ethic.
I agree Anebel.
I see a very stiff rider - holding her horses face with her hands and not stable in the saddle.
There is absolutely nothing wrong with going on the lunge line - I do it, many around here do it.....even GP riders go on the lunge. The Spanish Riding School - they are on the lunge line for a year with no reins.
You gain so much by being on the lunge line - and I am sure your coach would work with you.
You need to fine tune your seat, your legs. Estabishing balance and lower body strength. You need to learn to use your seat to control rhythm and tempo and your legs to aid your seat.
All this, your coach should beable to show you, work with you and help you with.
In the video, I see really stiff arms, really stiff shoulders and elbows. I see a rider holding the horses face. Right now, you are riding front first - understandably that you don't want to sit on your babies back.
I would do allot of work at the walk and trot. Work on balance, straitness, rhythm, riding back to front.
My Coach says, if you cannot do it at a walk, you have no business doing it at the trot. If you cannot do it at the trot, you have no business doing it at the canter.
Take your time - as already stated, You only know as much as your coach. And your horse only knows as much as you.
Our horses reflect us.
She is a beautiful mare, and the both of you have allot of potential - but I would be focusing on yourself first and really work on your solidity in the saddle before transferring that to your lovely baby girl.