Re-generate a past private key
There's no way to go backwards from the public key to the private key, that would mean that anyone could steal your Bitcoin.

You might be able to do something with that passkey you have saved though. Is it a list of words? Do you remember what app you were using?

Edit: Looks like you actually have a passkey that was used to generate a private key and not a Mnemonic backup. Have you tried putting the private key into the 'wallet details' section of this website: https://www.bitaddress.org/ ?

Note that you can use the site offline and I recommend it.