I can’t seem to balance when I perform Bulgarian split squats. Why?

You’re probably progressing too fast, too soon. The smaller your base of support, the more precise and accurate your postural adjustments need to be. First, master the basic split squat and then progress to having your foot on a low box. As you conquer that, increase the height of the box until you can balance on a bench. Another tip is to find a point of focus, like a pole, and use that to help keep you balanced. 

Pro Tip: Geremia suggests looking away from mirrors during the move, as they can disrupt focus and affect balance.