If this has been mentioned somewhere else and I've missed it, my apologies.
Selling horses, while earning you simoleans, does not count toward The Jockey LTW. I've had the wish since starting my toon as well as self-employed Horseman. My total earned for Horseman is 75,515 while my total currently toward The Jockey is 26,000. I'm assuming that's from strictly racing. I haven't put my stud up for service at the Equestrian Center as I've been breeding at home and selling the foals once they age up. I've also made sure that my LTW sim has been the one to sell the horses. I guess I'll have to do the Stud Service at the center if I want to speed it up.