#1 Lesson I Learned as a (Successful) Trader#1 Lesson that will keep you profitable: SELL BEFORE IT'S TOO LATE
Translation: Always Use Stop Limit Order to 'limit' your losses when you're wrong or when market decides to prove you wrong. Market is neither efficient, nor smart. Index funds buy and sell stocks on a daily basis to match corresponding benchmarks -- they don't care about fundamental or technical analysis of your stock (if they own your stock, they will buy more or sell more to maintain benchmark returns for their respective indexes). Be smart. Stay safe.