If you only have the private posting key, there is no way to recover your account or change its master password. You are lucky that it happens now to you, where your account does not hold a big amount of STEEM/SP.

So, you need to create a new account. The lesson you learned is: Always store your master password in a safe place! I strongly recommend to also keep an offline copy on paper.

If you need help for creating your new account, just let me know.