Password Vault Storing Passwords

I’ve been working on just a fun side project in C++ to practice encryption algorithms and I sort of ran into a bit of a roadblock of sorts. To summarize the project I have a text file of encrypted passwords and sites the passwords are for, the user logs in and the encrypted information is stored into a list. Simple enough. The log in works just fine and is as close to using a hash function as I care to try for as small a project as I’d like this to be. My main problem is this :

When I want to show the user their password, I need to decrypt it. How do I pass the key to the decryption algorithm without having it stored in at least RAM? Because I have a encrypted version of the key stored in the text file as well and relies on the hash being correct for the key to be decrypted. But I don’t want the user to enter their information again to get the password after they already logged in. So how should I go about this? Should I save the key once I can decrypt it with the users information? Or should is there some solution that is escaping me? Thanks for any help!