What did I do wrong that caused me to lose bitcoin?
1) If you generate your paper wallet on your computer, you have to fully trust your computer

2) If you use a website for generating your paper wallet, a) you need to trust that website (hopefully its local javascript) b) your browser and its random number generator and c) eventually your network provider(s) (depending on the transport layer security)

3) If you generate your paper wallet on your computer and it was compromised, the generated paper wallet was accessible at least during the time of your generation.

My recommendation

1. Never ever use a online paperwallet generator

2. Hardware wallets are eventually not super secure, but they offer great security, maybe use them instead of a paperwallet

3. The BIP39 24 word seed can be restored even if the vendor of your HWW disappears

Conclusion:

Use an old offline computer with Bitcoin Core or any other semi-trusted software to generate your private keys or seeds, keep it offline all the time!

Or... use multiple hardware wallets from different vendors in a multisig setup if you want maximal security (but comes with minimal user experience)