As a writer myself, I can see where you'd like the idea of the hero lifting his love from the back of her horse onto his horse...but in all practicality, unless she's a midget and he's built like Dwayne Johnson or Arnold Schwartzenegger, it ain't gonna happen. At least not without some major hernia action going on.
Without actually reading the scene, it's difficult to suggest an alternate course for the scene to take. I'll do my best, though.
After your characters reach the end of their race, maybe have your hero complain that the heroine won unfairly because his horse had a stone in its hoof. He dismounts to prove his point (there is no stone, but being the hero, he has to make *some* excuse
) and the heroine starts to dismount to join him. He moves to help her dismount (what self-respecting hero wouldn't?!). She swings her leg over the front of the saddle (assuming she's riding astride), he grabs her waist and lowers her to stand so that she's effectively trapped between her horse and him. He then leans down and kisses her...yadda yadda yadda...etc, etc, etc.
^^This is just a rough sketch/idea. Again, without reading what you already have written, I don't know how well it would fit or anything.