A good piece of advice is to think of the turns as circles and think of the circle as a clock. Imagine that the start of the turn is at twelve o'clock, while the end of the turn occurs between six o'clock and eight o'clock depending on how sharp a turn you make.
One o'clock.
Five o'clock.
When you have come half way around the turn, at about three o'clock, your skis should point downhill. It is at this point that you have the greatest need to control your speed. Unfortunately, this is difficult to do as long as your skis are pointing directly down the slope. Only after you have passed the point where the skis are pointing straight downhill, is it possible to work on controlling your speed again. You are in a no-control zone, between about two o'clock to four o'clock.
Three o'clock.
You have probably experienced a similar situation when for example you drive a car in slippery conditions. When slowing the car down, there is a natural tendency to brake hard. However the exact opposite happens. The brakes lock, and the car begins to skid without you having any control over the direction or speed. The same thing happens when skiing. If you try to brake by forcefully pushing skis across the no-control zone around three o'clock, external forces will continue to pull you down the slope, resulting in bad balance, or in the worst case, a fall.
The best solution to this dilemma is to have patience. When you are in the no-control zone, stay calm and continue the arc of the turn that you have already started. When you calmly continue the arc of the turn, you end up skiing across the slope or even slightly uphill. This will reduce your speed and give you a better balance