Frustrated with inability to do body work!

Ive been working on the front fenders of my 66 dart on and off for 6 weeks. Im no farther ahead tonight then I was at the beginning. Problem Im having isnt making surface smooth but its creating or should I say recreating the body lines. Heres the history, My dart is a 270, it had a body trim that went from front to rear about 3/4 of the way towards the top of the fenders and door and rear quarters. Ive removed them and will not be using them again so I had to weld the holes that are in the body to hold trim in place. Here is a pic of the fender.
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Above the center between the 2 indents are the biggest issue. There are 3 places that you can see I have put filler into. My issue is remaking that body line both top and bottom. First I am struggling to find the exact line to follow. Ive been trying to stretch tape along the old line but its very hard to see that exact line since the line is rounded toward the indents. I can make the indented area flat but im struggling to then match the rounded body line to the flat area without going to high into the center area or to low into the flat indented area. I feel like all i do is make filler and sand it off and repeat! I tried using 180 grit and also 80 grit on sanding blocks both flat and rounded ones.
View attachment 1715083200 Above you can see I have it somewhat filled in but it was to low at the flat area of the indent. Below it looks like I have body lines but they werent straight. Any suggestions on a good method would be greatly appreciated.
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I want to do this myself and not have to pay someone to do it but so far I am going nowhere fast!
Thanks Rod
You're taking too much material back off. Work the front half of the fender as one piece. Coat the whole thing. Work the the higher section, flatter parts, then tape off the scallops and work the lower part/scallops. While sanding if you hit metal it's a high spot. Stop and recoat. I stay with 80g until I'm very close then skim coat with metal glaze, a partial knockdown with 80g to "de-skin" the plastic and onto 180g. Using too fine a paper too early will only waffle it up.

That's a pretty intricate piece for a newbie to tackle. I've been doing it for a long time and still use guidecoat on filler and tape bodylines to watch what I'm doing. I also start cutting when the filler is still a little soft as it speeds up the process. The quality of filler can make a difference. Some shrink and you can really chase your tail trying to make stuff nice.