Endgame -- I Suck

[EasilyConfused]

I just started playing $20 NL 1-table tourneys on Stars about a month ago after a couple years as a moderate winner in MTTs and ring games.

I've now played a little over 200 sit-&-gos, and I'm down $80. I have more than my share of 2nd & 3rd place finishes, but not nearly enough wins. I'm sure I'm doing something wrong when I get heads-up, but I don't know what.

Can someone kindly point me to a few previous helpful threads on endgame strategy so I can brush up?

I apologize if this is in the FAQ. If it is, I didn't see it.