The main thing which seems off to me is the dynamism and the weighting. In both things your reference is great.
I don't know a lot about how to ride a horse. In the reference the rider though seems to be much smaller than your girl and he sits closer towards the shoulders of the horse and not at the haunches.
The rider also seems to try to counterbalance the weight, although he pulls the horse's neck back because otherwise he would be thrown off.
If you don't go for the pulling part, the overall angle of the horse should be lower. Otherwise counterbalance the weighting of your figure by referencing the relative spine angles - there is a lot of difference.
Also look how the legs are wrapped around the horse's ribcage and how the upper and lower leg of the rider look.
Another thing which seems to be off is the structure of the legs of your horse. It's missing important joints. Either check up how the basic bone design for arms and legs looks like or look carefully at the reference.
In terms of general proportion your horses rear legs appear by far to long, compared to the whole body and also if you compare them against the length of the front legs the difference is huge.
The by far poorest part nonetheless is how the horses head looks. It's completely flat, seen from the side and don't conveys any form. In the reference you really can feel that you look up to the horses head and you can see the bottom plane and the side plane quite well.
well I actually just roughly tried to fix some proportions, the position of the rider and the weight distribution. l also tried to illustrate roughly what I mean with the head and the legs.
It's sketchy and not exact, but I have no time for making it better/clearer. Maybe it helps you to compare the spots I mentioned with the reference.