What’s the biggest mistake you made in an investment decision that has costed losses?

I’m 19 and recently picked up doing investments but I just lost $500 yesterday due to my own foolishness and greed. (Shorted S&P 500 index at the dip on Monday with 10x margin)

I’ve learned a lot and accepted that it was my own fault, but it just sucks that it had to cost me half of my 2021 savings to truly learn. From now on Im going to try a different approach by investing in long term without margin and not trying to be clever by predicting the market.

I couldnt sleep well last night due to the loss that I’ve gotten, but now I think I’m starting to be able accept it more. So I’m just curious if anyone has any story about their own past mistakes or foolishness in investing and also the lessons they learned from it which they are willing to share. Maybe I could learn a thing or two from others so I wont do similar mistakes in the future.

Your advice would be invaluable and thanks for listening.