the most important lesson I've learnt is to never trust those who tell me which altcoin is "hot" right now and also to never bag hold any altcoin for more than their pump can last. for most coins that I trade it is as short as a day since that is how long pumpers are going to pump that coin but it can go as big as a week mainly for bigger coins that don't get dumped that easily but get dumped all the same.
this shaped up my overall strategy. I accumulate bitcoin (long term investment) while short term trade altcoins to increase the number of bitcoins I own.
That is a great lesson you have learned! I have see this also in some of the ico I invested in. Some of them has the best rating in some of the rating sites and huge funds raised and on social media, we do have the promotion of them but at the end, if you invest in them you will find out that it does not worth to be invested in. There are many tokens that have lost over 97% of ico price and sometimes I do ask myself if people really put in funds into those projects or I was the only one that has been deceiving in.