It looks like you are well on your way, just need to keep at it. If you can find a flexible sanding block, that will help. Perhaps a piece of 1/4" hard rubber that you could attach the sandpaper to. Another helper is to take a large dowel (1", like a piece from a broom stick) and wrap the sandpaper around it. When you get close enough, then fill the seam (spakling compound, or sawdust hardened w/ CA) and sand smooth.