No, I don't think it's just one thing either, with all your experience in so many conditions. I suspect your techniical skills are well up to the challenge of powder. In fact, real powder--that rare, untracked, marshmallow covering of deep or bottomless snow--is really one of the easiest conditions to ski. It can make technically poor skiers look, and feel, like heros. So I'm sure you can do it too!
What is likely is simply that you lack the mileage in powder that will enable you to relax and ski the way you're capable of skiing. Powder--especially deep powder--is a very different environment than most other conditions. It looks different--and you may not be able to see your skis, which can be disconcerting until you get used to it. (Trust me--they're still down there--and you'd know it right away if they weren't!)
More importantly, it feels
different. All the sensations that you've learned to expect and react to, that give your body the feedback it needs to perform with skill and polish, and that tell you you're in control, feel different. You don't ski on your edges--you ski on the bottom of your feet, no matter how much your skis are tipped. You float in the the snow, not on it. When you extend your legs, you don't get taller--your feet move lower. There's nothing to "push off" from, and when you start to sense a loss of balance, there's nothing to brace yourself against, or to push on with your feet or poles to recover. And there's no sound--no grating of your edges, or thumping on bumps. Sensations and sounds are all more subtle--it may feel like you're in a sensory deprivation chamber. Everything feels unfamiliar, and your body may react by tensing up, just when it needs to be loose and flexible and sensitive.
But it's not difficult. You just need to get used to it. Yes, you do need a bit of speed in powder. "Speed is your friend," they say. But you really don't need much, especially with today's wider skis. Unfortunately, like water skiing, you can't start out really slowly and expect much success, and you have to find a slope with enough pitch to get a little speed on. A typical green learning run is not steep enough!
So, the next time it snows, find a gentle blue run. Ideally, you'll look for powder that is not truly "bottomless"--maybe 4 to 6 inches of snow that still gives you a solid base of support when you need it--and adds confidence just knowing it's there. But don't hesitate to go out, no matter how much snow there is. Start in a shallow traverse, just steep enough to allow you to gain speed slowly as you glide straight through it. Stand centered fore-and-aft and on both feet, and feel the sensations as your skis start to float up with a little speed. Relax--but not completely. Keep a little tension in your core, a little functional tension throughout your body, which you need for the athletic movements of balancing, especially if the snow is at all uneven. Notice that you don't gain nearly as much speed as you would in the same traverse on harder snow. Powder is slow! Arc uphill to a stop. (Don't try to hit the brakes. The last thing you want in powder is skis going sideways!)
Do it again, this time flexing up and down, allowing the skis to "porpoise" as they float through the snow. Try to find the snow's natural rhythm, like bouncing on a diving board or a trampoline. Make sure you're breathing--easily and naturally (it's common to hold your breath--make sure you aren't!). It shouldn't take you long to learn to enjoy this sensation, and get comfortable with it. Swing your poles forward and back in synch with the rhythm. Don't try to plant them--there's nothing to plant them in--just swing them, fluidly and loosely, with your wrists and fingers.
Next, try very gentle, rhythmic direction changes as you bob up and down. When you stop, look back at your tracks to see the tell-tale "S" curves of powder skiing! You'll find that it doesn't take much turning to slow you down--a lot. Soon, you'll learn to trust that you can take a much straighter line in powder than you can on firmer, faster snow. When you're comfortable with this, go a little steeper, a little faster. It only gets easier--as long as you stay within your comfort zone. The faster you go, the more you'll feel your skis float up beneath you as you relax, and plunge back down as you extend your legs into the powder. Assuming your skis are soft enough and long enough, they'll bend into an arc and help you shape the turn, just like carving on hard snow. Keep the rhythm going--never stop moving--keep swinging your poles.
Keep your upper body and arms as still as possible, letting your legs flex and extend and rotate beneath you. It may be tempting to try to force your skis around by rotating your upper body, but seek to do the work with your legs, letting your skis arc and slice through the snow. As much as possible, keep them going the direction they're pointing, rather than trying to force them to go sideways. This may be easier said than done, because forcing them sideways is our natural defensive instinct. And, frankly, it will get the job done in powder. It's how most skiers ski it--powerful upper body rotation, skis never running far from underneath their bodies--generally poor and inefficient technique, but not entirely ineffective in powder. It's the image that pervades many powder skiing videos and movies, so it may be ingrained in your mind.
But don't do it! Let your skis run. If you find yourself becoming defensive, go back to a shallower slope or traverse, and refind that sensuous, floating, gliding, slicing feeling of skis going the direction they're pointed. Then come back to the smooth, rhythmic movements of your legs that are all it takes to guide your skis and get them to do what they're designed to do. It really doesn't take much effort, or much movement. You'll find that, as your confidence grows and you get comfortable with more speed and with the unique sensations of skiing powder, you'll need to do less and less work. .
Give some attention to the shape of your turns. They should be round--not z's. Make sure that you're aware of at least a moment in each turn when your skis are pointed straight downhill, and you're letting them run that way. Be patient--they'll come around!
It will not take you long, I promise! After playinig with a few of these exercises to get the feel of powder, see if you can find a friend to ski with--someone comfortable in powder. Let your partner set the rhythm, while you practice skiing in sync, making parallel "spooned" tracks, or figure-8's right behind. Take turns in the lead, too, focusing on setting a smooth, consistent rhythm that's easy for your partner to follow.
That's it! If you've gotten this far, you're a confirmed powder hound by now. Start saving for that helicopter trip!