one question I haven't seen asked is....what grit sand paper were you using?
To start off with you need to use a grittier sand paper, then move on to lighter sand paper, it will make the sanding process go a bit smoother.
Also, a little tip/trick....once you have the bondo in place and it's starting to kick, start sanding lightly before it's funny kicked....this is an old body shop trick. They tend to use something they refer to as a cheese grater, you can get away with just using some 100 grit sand paper...
it looks like you need to work smarter, rather than harder
keep it up!