I know the username and password, I forgot the instance. I registered through an app so the login details were saved on my keychain but the keychain points to the app, not the instance.

I'm not saying I can't figure it out and do it better next time, but on Twitter if you forget your password you just push a button and then reset it. On mastodon if you forget your instance you're SOL. With Lemmy I know I'm on the big main instance and it's not a concern anymore.