I can´t play truly shorthanded poker

[Apanage]

When Tryptamen asked about filtered stats in the other thread I looked in my databases and realized that I really really suck at playing Heads up and threehanded.
I´m Having a winrate of 1.5 BB/100 overall with filtered for 2-6 players. But filtered for 2-3 handed I have a winrate of -0.76 BB/100. This can´t just be a rake issue can it?
3-handed I play as if every hand would have been folded to the button. Is this a mistake?And if so in which position is it the biggest mistake?
And if one would learn to play heads up where would one start? The heads up forum at 2+2 is no good since it mixes up the different poker forms.