Decrypting SSL traffic over a Proxy

I would like to decrypt TLS traffic. Normally I know how to do this, but in this case I do not have a direct connection to the server. Instead, I need to access the server over a web proxy.

My proxy listens on port 8080. My client performs an HTTP CONNECT to the proxy to connect with port 443 of my server. After that, the SSL traffic between client and server is tunneled.

Diffie-Hellman is deactivated; the Cipher Suite is TLS_RSA_WITH_AES_256_CBC_SHA. I have the Certificate with Private Key in the RSA keys list. In this list, I have the IP address and port of the proxy.

However, Wireshark doesn't decrypt the traffic and I don't know why. Is it possible to decrypt traffic with this setup?