This seems along the lines of ECIES
http://en.wikipedia.org/wiki/Integrated_Encryption_SchemeI'm not sure exactly how your system works but I would have thought it should be possible to use ECDH to create a shared secret without compromising your secret key and use this in a symmetric encryption scheme (AES) without compromising the shared secret? Isn't this basically what is done in ECIES?
The private key is used in the creation of the shared secret but if this is done ok then using the shared secret in a secure symmetric system shouldn't compromise it in any way.
I may have misunderstood things but I don't see how the same private key is used for signing and encrypting - the shared secret used for encrypting isn't the private key of the bitcoin address is it??