Also be sure you have click-to-move enabled. Which doesnt have to do with mounting, but can cause the walk backwards thing.
Make sure your mount is spelled correctly in your macro? I dont see why this would be happening. Its a very rare issue. If restarting GB/WoW doesnt fix it then I dont know. Make sure its on hotbar 3? I made mistake of doing it on 4 first itme I tried.