I'm especially fond of the last line:
It is your heart that this horse is stealing!
Being a horseowner myself--I know exactly what you mean. You may want to consider showing us one moment, even running through the grassy field... it's almost too broad. Try importing some sensory imagery into your work. The feel of the horse underneathe you, the muscles, the smell of grass and dirt being trod up as you are riding... the sound of the hooves, and try to focus those things like light through a magnifying lens into one particular moment or memory, of riding.
You'll find by incorporating the concrete details of experience, that your poetry is even more effective, and rich.
Thank you for this,