I've never done much more than a sanding job on a J2P, so take my advice with a grain of salt.
I know how you feel about filling and sanding a lot... I recently broke my own helmet, and I honestly spent two or three days applying JB-Weld, Bondo, and glue, and sanding it all down. You just need to be patient, and keep at it. However... There has to a point where you say, "I'm done with the body work, it's time to finish it."
There's no way on earth to be too picky about building a costume. If you want to make everything out of bullet proof metal and Kevlar, yeah, that's a little picky... But wanting your helmet to look as good as possible is completely normal.
To my basic knowledge, you're not doing anything wrong... You just might want to step back for a day, and do some thinking on what you're doing/what else you can do.
With as much filling/sanding you've done at this point, I'm tempted to say no. If you're too frustrated with it, and you just want to start over, go ahead. However, I'd like to impress upon you the need to stop, breath, and think about what you want to do with it.
