The hardest part is selling stocks. Many get stuck in thinking they will always recover, or dream of the Microsoft type stock that gives a huge return over decades but that rarely happens. Most stocks over and under shoot valuations. At a minimum it is best to trim on the way up and add on the way down with the caveat that buying on the way down should only be done if the stock really has a solid financial footing and not a fad stock.
Even with a stock like Berkshire that rose to ~$350s and mungofitch mentioned it was getting up there in terms of fully valued. Now it is back to ~$300s.
Of course right now most things are just losing money unless commodities or housing. Stocks, bonds, cash, etc. losing money to inflation or just to markets or both. Painful for many.