I have some suggestions in addition to the good ones that were already made. These come from sports psychology. It's human nature to focus on the bad things that might happen, but as you've found out, that tends to snowball. So make an effort to focus on the positives as much as the negatives. You've had this horse for 11 years and this is the only time this has happened? Plus, the circumstances were terrible. It's no wonder your poor horse spooked. But it hasn't happened again. Every time you have a good ride, make a point of going over it in your mind right afterward and think about the things that worked well.
If you're worried before you even get on your horse, do this: "He might spook today, it's windy. THEREFORE I'll either lunge him first so I can tell how flighty he might be, or ride less aggressively than I usually do, or really focus on keeping my body position perfect and breathing well, or if I might just do groundwork today." Then remind yourself, "But probably everything will be fine."
This might sound weird, but do you jump at all? My instructor pointed out that jumping has some of the same motions or issues that you can use to stay on a spooky horse. And finally--my standard advice to everybody for everything--lunge lessons will help you improve your seat. I know you might not have all these options in your area, but if you have any of them, you could think about it. Good luck!