It is far better to learn from other people’s investment mistakes than learning from your own. I’ll start:
Influenced by the share price movement
Changing my narrative to justify the reason for holding i.e why a holding is still undervalued after +100% or why a holding is deeply undervalued after a -80% decline despite changes to the fundamental.
No idea what I’m buying
If you don’t know what you’re getting yourself into, chances are you won’t have a clue when to get out. The latter is more important. As Nassim Taleb used to say 'Better to take risks you can measure than measuring the risk you’re taking." Bought GTN at $2.06 in 2016 right after IPO, went up to $3.90 (+89%) in 4 months feeling I knew it all along (when all I know was they do radio ads while reporting traffic conditions from the helicopter). Currently trading at $0.48.
Not sticking to my rule
When I bought Magellan (MFG) during COVID (avg. cost base $32+), I made a selling rule that I’ll sell if Hamish Douglass quit the company (because I wasn’t sure if MFG’s outstanding record is because of individual brilliance or its culture or something else like macro tailwinds). He did. And I didn’t sell for whatever self-justified reasons. Currently trading at $8.53