Your username is from lemmy.world so just go to https://lemmy.world/ and that will be where you would log in

The “instance” is where your user is held - that’s lemmy.world. Any other instance, like lemmy.ml or sh.itjust.works or any of that won’t let you log in because those instances don’t have your user in their databases, but you can interact with them from your log in through lemmy.world