USE YOUR WORDS.
Really, it works. ;) Just approach them politely and say something like, "Hi, I really appreciate the opportunity that you've provided me, but I have a few concerns. Is this a good time to discuss them?" And if said person says yes, voice your concerns.
"I've been working very hard, and I feel disappointed, because over the past month that we've had this deal, I've only been allowed to ride the horse once, and for around five minutes. I understand that everyone is very busy, but I feel like I keep getting put off; I'm working very hard and would really enjoy being able to spend more time with and riding -insert horse name here-." And then discuss!