You can neither respec nor start over. The abilities are character abilities ("horse riding training"), not horse properties. This got changed some patches ago (I think in conjunction with 1.6.5, but not sure).
But there is no need. You can maximize all three stats. Just stop doing one and switch over to the other. It may take a while but you will get there. If you can't be bothered to wait, you have the option to use the crown store lessons to speed up the process.