Catch 22What Pierre said.
You can learn to ski in powder through mileage and negative feedback. It's just not very efficient and it's frustrating.
You can learn to ski in powder from the Internet. It's just not very efficient because we're guessing too much when we give advice and the advice you get needs some filtering and prioritizing. That said I'm going to try to help anyway.
Bad hand positions can pull the body out of position, but good body positions can encourage the hands to be in a good position. In your case, to break this catch 22 you need to learn (or relearn) body movements to the inside of the new turn. There are 1000's of ways to do this. These drill suggestions probably aren't the best ones for you, but they are presented to give you ideas. Do them on groomers, then take the movements to the powder.
Heisman - This drill involves using a position similar to the Heisman trophy (one hand on your hip carry ring the football and your inside hand leading forward to stiff arm your opponent). No poles for this drill! The key to this drill is to change edges first, then switch hands to move the new inside hand to the stiff arm position. The objective for this drill is to get you in a good countered position before you start your next turn.
(Thanks Epic for the politically correct name for this)
But this is not going to help if you don't start your turns by moving your core into the new turn.
Flamingo turn - this is where you lift the new inside ski tail and tip and pressure the ski tip into the new turn. In lots of lessons, there is a progression of steps starting from lifting the inside ski, through getting it level or tip lowered, then pressured, etc. The objective of this drill is to start drawing the core of the body into the new turn earlier in the turn.
(I made this name up to avoid using someone else's name that was slapped on to a new version of an old old exercise)
Ankle Raise - We talk about old inside leg extension occasionally, but a key component that's missing from a lot of skiers is opening the ankle joint. This exercise asks to focus on just extending the old inside ankle to drive the start of the new turn. Like the Flamingo turn, there are a host of progression lead ins to develop the ability and awareness of ankle movement (e.g. on a flat run hop the heels up 2" without changing leg or knee bend). The end objective here is to move the hips from inside the old turn to inside the new turn keeping them at the same height off the snow at all times. Some people can do this automatically just by focusing on keeping the hips level. Some people can do this by only focusing on flexing the new inside leg to start the new turn. But most skiers need to add this old inside ankle movement to put it all together.
(with thanks to Mac Jackson - we need a better name for this)
White Pass - This drill is where you finish a turn standing only on your outside ski, then start the next turn doing the edge change on that ski. After you pass the fall line, you set your new outside ski down, then continue to make the next edge change on that ski. You need to do this drill until it becomes ridiculously easy, but if you don't have the movements this drill will not teach you the movements. One key to this drill is that the raised leg has to continue flexing and extending while off the snow. The end result of this drill is that movement of the core into the new turn gets burned into the brain as something to be done no matter what. (with thanks to the Mahre brothers )
Heisman (again) - OK - now that you've got the moving inside movements locked in, putting yourself in a good position to make those movements will make a lot more sense now.
It may seem weird that I'm asking you to fix your inside hand back problem simply by moving it forward. Sorry - it's the catch 22 thing. What we're really trying to do is change the way you start turns so that you don't finish in a difficult position. Focus on moving that hand forward starting from the middle of the turn (in the fall line).
Some of the other ways mentioned in this thread could possibly work much more effectively. The "slow motion" tip often has great success. Similar tips for deeper snow (that are not necessary with fatter skis) include raising your hands higher in the air after a pole touch to induce more bounce; focusing on skiing 3 dimensionally within the snow where the low depth occurs in the fall line and high point off the bottom occurs at the transition (across the fall line); and using a slightly narrower stance to get both legs acting as one unit instead of independently. That last tip may seem contradictory to the set of drills above. Yogi Berra says you won't get it till you get it.