Bitcoin public key to private key
This is simply practically not possible; it's one of the basic premises of public-key cryptography:


  The strength of a public key cryptography system relies on the computational effort (work factor in cryptography) required to find the private key from its paired public key. If so, effective security only requires keeping the private key private; the public key can be openly distributed without compromising security.


Unless somebody breaks the cryptographic algorithm used to generate the keys, your best bet is to simply generate random keys and check whether the corresponding public key matches the one you already have. (Well, that's a slight exaggeration but you get the point.)