The first two are the most important concepts in investing yet there are so many people that think they do it but don't.

The third is a big misconception...if your portfolio is down 20% you lost 20% of your investment whether you sell or not. Think about it, if someone wants to mimic your portfolio it is just a matter of buying the same that you did but paying less. That is what is called the mark to market valuation, unfortunately the value is what the market is willing to pay right now, not a year or a day ago. It is ok if you are holding long term to not get obsessed with the mark to market value of a portfolio but for all means the value is what is worth today not the entry price and for that matter the entry price becomes irrelevant to the valuation of the portfolio at the moment you do the purchase.