You will need something like the following…
openssl pkcs12 -in x.pfx -nocerts -nodes -passin pass:123456 | openssl rsa -out privkey.pem
Replacing x.pfx with the name of your pfx file, 123456 with your certificate’s password, and privkey.pem with the name you want for your output file. You can then open the privkey.pem (or whatever you called it) and that will have your private key.