Filly

To start with, I really do like this!
I like:

The post: It's a really dynamic pose, and it looks really elegant! Many people find this sort of thing very hard to do with heavy horses, but you manage just fine!
The colouring: The markings are just divine, and the subtle shading just set's the picture on fire!
The Background: The shape of the background is really unique and it works very well!
Hair: I do also love the gradients in the main and tail, as well as the neatness of the tail. It's perfectly cut, and many people struggle with getting uniformly cut tails to look effective, but you've nailed it!

If I was to suggest one improvement, it would be to shade the hooves, and maybe add some grey into the socks/ stockings.

Overall, a truly beautiful picture!