Hello Alan,
This exact subject has been a problem of mine for a long time as well. Yes it is a gin you are a little passive if your playing short handed, as you should beable to keep your non-showdown winnings around you breakeven point. The best way i found to counter this is 4betting lighter pre flop, i know this can be harder at the lower stakes so choose your opponents wisely. stealing and 4bet re-stealing the button the cut off, even blind v blind is the way to counter this problem imo.
Remember 4betting a re-steal puts all the pressure on your opponent as you may only be commiting 25bbs whereas if he wants to play he only has two options, playing oop against you in a 4bet pot or 5bet shoving for 100bb, either way he is at a disposition without a massive hand.
And like others said you can get a little more aggressive post-flop, but that depends on your hand reading skills and your stats analysis.
Anyway i play full ring and have always had a steady negative non-showdown winnings graph, on full ring i find it very hard to counteract this, as players dont 3bet as light as short handed and its much more of a grind. AS you can see though its not all that bad, as long as your making correct calls and lay downs you'll see decent profit!

Full ring 50-200nl