This isn't entirely true as you can also learn form others experience. There's a popular saying that goes this way "wise traders learn from their mistakes, perfect them and turned their previous experience into a profitable one but smart traders don't wait to make this mistakes themselves, they learn from others mistakes. This way the avoid losses although that doesn't mean if they do make their mistakes they won't learn from them.

Anyways my unforgettable experience are numerous to pick one but I'll go with the most recent which has to do with DeFi trading. I bought into a newly listed coin on Binance as I intended to catch the FOMO before it kick off but unfortunately I didn't research properly to know the newly listed coin has already done impressive ROI and was due for some retracing.

I'm still yet to recover from that bad choice of mine and it was as a result of my lack of proper researching and just jumping into a project because it was getting listed on a top exchange.