I've only sold stock short once in my life. It was Apple.
Mostly, I've just bought stock I thought were undervalued. My worst investment was a company called "Fortune Systems" that had a Unix implementation on IBM PCs in the 80's. Ran across it at a tech show. I was a big fan of Unix using it for dev work. What I didn't understand was that PC's were rarely used for multi-user work with a bunch of serial ports driving terminals. The market for their product was miniscule. It became obvious a few months later and I sold my shares at a 60% loss, about a 10k$ loss. It was worthless a year after that.
Then the SEC introduced EDGAR. The real time web database with company SEC filings. Anyone that invests w/o checking out EDGAR filings is just gambling.
As for the Apple short, I shorted about 50k$ around the time the PC was exploding and Apple was becoming less competitive and Jobs had left. Covered the short 6 months later for a 20k$ profit. I got lucky. Good thing, they got their act together and exploded a few years later.
The most money I ever made was going long for >$100k ish on MSTR after it had dropped about 99%. This was when Michael Saylor became the poster boy for bad financials after he tried to do a secondary offering and the SEC looked into his filings. Went from the local DC hero to zero in a few months. I followed but didn't trade the stock for several years as he got "toxic," Ie: convertible at market, private equity to stay in biz. This produced a classic death spiral in almost every company that did it.
However, Saylor actually had a decent business and worked hard. Turned it around in 3 years or so. After the Fortune System debacle, I started following MSTR's SEC filings religiously. I'd read the 10K and Q filings and watched the price just drop continuously.
Then something happened. MSTR converted the toxic financing, and their business started a slow improvement. Stock wasn't recovering. People took a look at the chart, which was a continuous decline to near zero and moved on. But the Form 4's and Proxy statement had something very interesting. The directors Saylor had agreed to add to provide "adult supervision" were not staying on. Saylor was forced to have them up to then. Obviously not very good relations between Saylor and them. So they were out.
But what was interesting is that they had exercised the options they earned as directors but, unlike most cases, had not sold them even though they were out of the company. Biggest positive flag I ever say. Not only did I invest 100k but I got some friends to do so too.
But I'm a short term person looking for a distinct edge. The stock went up and I closed out a few months later. If I held on to them they would be worth several million today.
As for today, Saylor has become THE corporate bitcoin evangelist and MSTR pretty much tracks bitcoin price. Weird.