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

Premium’s been a thing for a long time, it used to be Reddit Gold several years ago. It also used to be cheaper, $3.99/mo but it went up multiple years ago.

They might be pushing it hard right now, though. Not sure. Maybe they’re trying to entice the people who were paying for 3PA features to pay Reddit instead or something.

I currently have premium, ad free is the only way Reddit is palatable even before all this went down and I bought it ages ago when I wanted to support a thing I used every day and also have had a couple awards that extended it, it expires in August. I won’t be renewing.