Re-generate a past private key I know that every public key has a private key... I am wondering how to regenerate a private key that was generated by my Blockchain app (using a plain text passkey, which I am assuming was converted into the proper format by the app) 

I have the wallet address where the bitcoins were sent, but cannot use the plain text private key I used in the application to spend the bitcoins...

The bitcoins have been sitting at this address since 2015, And I guess I was hoping if there is a way to convert the passphrase I used in the app into the proper private key to access the bitcoins at the address they were sent to... 

I hope that this makes sense.... I can see them there, and not being able to access them is so frustrating! I lost the data in the application, I think due to a glitch. I was hoping since it is a mathematics thing to generate the private key that there must be a way to go backwards, using the public address and the plain text passkey I had used to find the proper format 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.