Less experienced skiers will often slide along in a comfortable, evenly-weighted stance, waiting for something to happen before they respond. While this may seem like a good strategy, you need to take a more dynamic approach when sliding over variable snow.
Don't wait for something to happen. Make it happen.
When the snow is fast and the cover is thin, you need to evaluate it all the time. Obviously you don't want to ski something steep and rough. But some parts of the lower-angled slopes you might choose will still have short, steeper drops within them. To negotiate these, you may not be able to maintain a wedge or snowplow position, because the trail is narrow or obstacles stick up to catch your skis. In that case, step from ski to ski, angling each ski inward before you transfer weight to it. Once you have weighted the ski, bring the other ski over parallel to it. If you start this sequence before you have accelerated to a speed that bothers you, the move will be easy and it will automatically control your speed for you.
Practice on a slope with no obstacles, so you can feel how it works without having to worry about navigation through hazards.
This stepping technique is the basis of jump turns on steeper terrain at higher speeds. As you become more comfortable with it you will find yourself doing it at faster speeds. The step and jump turns will evolve by themselves if you let yourself explore progressively steeper terrain. If you feel anxious, scale back to easier ground for a while. Just keep practicing, and don't let anyone force you into a dangerous or scary situation. Often the only thing that keeps us from mastering a skill is our own arbitrary timetable for the learning curve, or the pressure put on us by others to get "up to speed."