What really helped me at first was find someone at your barn to ride with. Schedule a ride after work, that way you wont want to cancel on them and you'll go. It worked with me. Even if you dont really know some one. What better way to get motivated then go to for a relaxing, stress free ride and talk about your horse the whole time.
And if it is too hot and buggy to ride, just go see your horse for a few minutes, give him a pat and some cookies. Being in the environment helps too. Once you're there you may want to hop on for 15 minutes just for a quick walk.
You just have to get yourself there.