I'm wondering if there's anyone out there who knows how to retrieve your private key using your 12-word passphrase? Your advice is much appreciated
There is no one private key associated with the 12 word mnemonic that you posses. Consider the seed phrase to be like a keychain, that holds multiple keys. You would need to import the 12 word mnemonic you possess into a wallet software in order to generate the private keys. The wallet software will then scan the Bitcoin blockchain in order to see the funds that those keys control. A use of a hardware wallet is most recommended in order to ensure the privacy of these keys.
Care should however, to use the same derivation path for the keys that was used when you generated the addresses to receive the funds. This can be looked up based on what wallet software you used while generating the 12 word mnemonic. There are multiple derivation paths that wallets use. Some use BIP 44 while others use BIP 49 or even BIP 84.
To derive the private key, you need to have this information before - whether the mnemonic is used purely as BIP39 or is further used to generate HD wallet using BIP32/BIP49/BIP84. The retrieval process differs for each of the above possibilities.
If the mnemonic is used purely as BIP39, that is a Bitcoin address is generated directly from the randomly generated used encoded into mnemonic, you need to follow below steps: a) Do n * 11 (where n is the number of mnemonic words). If it is 12, you will get 12 * 11 = 132. b) Calculate the number of seed bits using, 132 = l + l/32, which will give you 128 bit. c) Convert each of the mnemonic words into their equivalent 11 bit sequences (using a map into the dictionary defined per BIP39). d) Strip the last n - l bits (step a and b)
The remaining bits should be hashed with SHA 256 to get the private key can be encoded into hexadecimal or used in binary form. If you need to generate public key, you'll need to solve for secp256k1 ECC equation.
If the mnemonic was used further to generate the HD wallet using BIP32, you need to know the derivation path to the child element in the tree structure (BIP44 defines the industry standard for that). Once you have a derivation path, follow the below process: a) Pass the mnemonic into the PBKDF2 (using HMAC512) function with mnemonic as the key, and passphrase "mnemonic" b) Pass this root seed through the HMAC defined by BIP32 to generate each child element until you reach the target.