Greg, one thing I've found that helps greatly with sanding is to make your own small sanding sticks.
Use flat tooth picks and glue the wide end to your sandpaper with thick superglue (thin superglue is too thin and doesn't hold real well, although I did use up a useless bottle of the stuff I had hanging around).
When it's thoroughly dry, you can cut each stick off the sandpaper when you need it, and store the rest. I make them using 320, 400 and 600 grit, and these will work for most application. If you're doing NMF, you need some even smaller grit paper.
I use these for almost all smaller type sanding, and even on fuselage and upper/lower wing joints. They're small enough to help avoid sanding where you don't want to sand.
Another tip to make sanding-life easier is to mask adjacent areas with black electrical tape, rather than masking tape. It's tougher. It tends to leave a black gluey residue, but that normally wipes off easily by rubbing it with a finger or thumb. Oddly enough, it doesn't seem to want to wash off with water.
As for filling, I've mostly settled on gap-filling superglue. I used to use modeling putties of different brands, but they take a while to dry--sometimes overnight, and then frequently chip and crack when you sand.
Superglue isn't hard to sand if you don't wait too long. I usually apply it with the point of a straight pin or a safety pin I've bent to shape to form a handle. Depending on how thick the application, you can sand superglue anywhere from 10 to 60 minutes after it's applied.
When you think you're done sanding, hold the part up toward a light source, like the ceiling light, and turn it this way and that to catch the light reflections on the part and the seam. You'll be able to see areas that still need work easily if there are any.
I remember the first time I tried sanding, decades ago, and was amazed when, after painting, the seam simply disappeared. It was like--"enlightenment!"
Good luck with your sanding. Don't let it intimidate you. Just keep plugging away at it.