Don’t be concerned about controlling the horse with the reins. When you are establishing “forward” in the horse, the only rein you usually need is an opening rein. Remember you will be riding circles (page 79, Forward: Riding with Eloise King) for the most part when you start your mounted work or you will be using the arena wall or fence (page 82) as a soft aid. You should not need an outside rein in the initial stages of training or retraining the horse.
Some people say that you need to support the outside shoulder with that outside rein. Losing that outside shoulder generally comes about when you do not have forward established in the horse and/or are over-aiding him. Go back to your basics; forward, check your position, etc., until the horse is strong and supple enough that the outside rein is no longer a blocking aid (which will pop that outside shoulder) and becomes the soft aid that it should be.
If you ever feel that you have lost control of the horse, go back to your circle. Twist your shoulders in the direction you want to turn, use an opening rein to that side and step into the inside stirrup. Stay balanced on top of the horse and relax (especially your legs). Breathe and you will soon be enjoying the ride.