A horse rider develops a relationship with a woman in order to help overcome her fear of horses.

Completely agree with Lotcher. I would recommend you check out the formula section of the site to see a good outline for writing loglines. Along with Lotcher's comments your logline also doesn't have an event. Something needs to happen to your character which forces them to take action.

I don't understand how the protagonist can be described as a being a "horse rider" when they are also described as having a fear of horse riding. Does the protagonist have a mental disability and is seeking equine therapy? Or perhaps the protagonist was a former jockey and is recovering from falling off a horse mid-race and wants to shake off her newfound fear of horse riding?

Currently the character in your logline does make much sense and this is bad since character is one of the most crucial things to communicate in a logline. Once found a way to better describe your character you then need to consider how to best describe the story, I need more of an understanding as the details of the relationship with the "woman" since it is not explained how that is related to the protagonist's fear of horses.