My first big mistake: thinking a 'sure thing' was actually sure

Been trading stocks for about a year now, mostly small caps and some blue chips. My biggest lesson so far was early on, thinking I'd found a 'sure thing' in a biotech firm ($BTEC) with a drug in phase 3 trials. The chatter was immense, analysts were bullish, and I just jumped in without really doing my own deep dive into their financials beyond the headline news.

I sized way too large for my account, confident it would double on the announcement. When the trial results came back with mixed efficacy, the stock plummeted. I held on for dear life, averaging down a bit, thinking it had to recover. It didn't. Ended up taking a painful loss that wiped out several months of small wins. Taught me a hard lesson about due diligence, position sizing, and not letting market sentiment completely override your own research. Still stings a bit thinking about it.