You should check your items box regularly to see if your equipment needs repairing.
By mousing over each item in your item storage, you will be able to see their condition of repair (expressed as a percentage of their full condition).
Items will degrade on a daily basis and require regular repairs.
If items become completely broken (i.e. 0% condition) they will have no effect on your horse, they will not be able to be equipped to your horse and will be significantly more expensive to repair than if you had repaired it before it became broken.
The more expensive or higher the level of the item is, the more expensive the repair costs will be. Higher level items degrade less quickly than lower level items, thus do not need to be unequipped and repaired as frequently.
Items that become broken whilst your horse is wearing them will be unequipped and returned to your item storage for repair.
To repair items, select the "Repair" button at the bottom of the Item Storage window. Items that do not need to be repaired (i.e. those that are at 100% condition or treats) will be greyed out.
Select the item that you want to repair and you will be quoted a price to repair the item to 100% condition.
If you select yes and have enough game cash to cover the repair cost, the item will then be repaired to full condition.