I was about 15 minutes away from the barn when the sky started to get a little dark and it started sprinkling. I thought oh great (I was wearing a white shirt too!), and turned around to walk/trot back to the barn. I didn't want to rush because the grass was wet and she likes to rush back to the barn. About a minute later, the sky opened up and it started pouring and there was thunder and lightning. My mare freaked out and reared and started galloping towards the barn. We are still working on our brakes at the gallop (you can't really gallop in the ring) as she is an ex-racehorse. I was trying to slow her down while I only had 1 stirrup and could barely see because my glasses were covered in rain. I would've spun her in a circle, but the grass was so wet I thought she would fall. She galloped almost all the way back to the barn and probably undid at least a weeks work of training. I got back on in the indoor and finished the ride.
Lesson learned. Even if the sky looks nice- always check the weather before a ride.