Building a body line help needed

What size / shape sanding block are you using? Your block should be as long as possible, and at least long enough to span the three low spots you're trying to fill in. I'd recommend you wrap your sandpaper around a wood dowel or piece of PVC pipe to remake the radius along the top and bottom, then you can finish the flat part in the center pretty easily with a long flat block. Get creative with what you use for sanding blocks, it's amazing what you'll find around the garage.
Right now it looks like you're getting too much filler build-up above your welded spot. Unless you really warped or dented your fender you shouldn't have much filler there when you're done. I know what you mean when you feel like all you're doing is putting filler on and sanding it all off.
I am by no means a professional body man but I've gotten good results with the above recommendations and a lot of patience.
Good luck!
Scott