Well, my big horror story isn't about CFDs but I think it's probably applicable.
I got interested in the cryptocurrency called Ethereum (ETH) just after it made its explosive push to $900 USD. Had it been on my radar even two months earlier, I could have picked up a few thousand and retired when it shot up, but....no point in crying about it.
Anyway, I invested some money. Did well trading on $20 and $30 daily price swings. Thought I was SUPER clever as the price kept nosing higher.
And then....well, if you follow the cryptocurrency markets at all, you know what happened. ETH lost about 90% of its value...about three days after I'd taken out a loan to buy more...class case of buy high and sell low, except I didn't sell. I should have. I should have sold at whatever price I could get to cut my losses and reinvest when it dropped to eighty bucks or so, but....I was stupid. So I held my stake and bled red ink for two years.
Then, I did something that's typically a no-no. I doubled down when it hit eighty bucks, buying more. Normally, it's a spectacularly bad idea to send good money chasing after bad but I really believed in the underlying tech behind ETH, so I just kept buying, which slowly brought my average price per ETH down.
Recently, it started trending back up, so after two years in the wilderness, I finally broke even and have now turned a tidy profit. Even so....I just about gave myself ulcers over those two years and ultimately had to just pretend the account didn't exist, save for once a month check ins on the price.
I made a BOATLOAD of mistakes during all that. I can't swear that I'll never repeat them, but I'm sure gonna try hard not to. That was brutal.