I'm being told my 12 words are wrong

In 2013 I bought bitcoin lol. I paid cash and used bitinstant for coinapult to deliver it, and I claimed it.

From there things get murky - I received a "Welcome To My Wallet" email from Blockchain. Presumably I followed the links and was given a 12-word seed phrase, which I have copied in multiple locations.

At some point I created a wallet with Bitcoin-Qt, and I got a dat file.

Then I ignored everything for a few years.

Recently, I've opened my dat file in Bitcoin Core and synced. I have a balance and I can see the date of my only transaction, but when I try to transact I am asked for a passphrase.

I have no memory of ever being required to make a passphrase for Qt. I try my 12-words anyway and I get denied.

On Blockchain, where I recently verified my identity, I get an email confirming that my address is linked to my wallet created back in 2013. However, when I try to recover it with my 12 words I get denied there as well.

Anyway, that's the story. Obviously I'm interested in regaining access, but I understand that I'm operating in a system that I don't understand. Any guidance would be much appreciated. Thank you.