This looks like an A-body fender(?) Are there any buckles above the area worked? If so were they spooned as you worked the dent? It appears the upper body line is still driven in or nonexistent (flattened), this will dictate the damaged area. It also looks like the body line below the dent and forward of the bracing, is flat. What you have is a flat area where the body line used to be and a buckle below it. (see photo)
Is there damage to the edge and a 'V' type dent just forward of the edge? If that edge is driven forward then the panel may be 'short' in the area of damage, also adding to your frustration.