it is much easier to critique a painting than to do one.
however, it takes many, many paintings to become good. so, hopefully , you will be excited enough about this one to go on and do some more!
parts of it are very good. the hind end, and parts of the Jockey are well excecuted. the thing that strikes me is that the horse's bone structure is a bit "mushy". I mean, the joints do not have the 'heft' that they should, and the leg that is bearing all the weight, (his back left) should have a fetlock that is quite flexed to show how the weight sinks down through this joint. People often make the knees and hocks of a hrose much too smooth and elegant, and as such, not believeable.
The color choices for your background are intrigueing, the blue and gold.