Why do my brakes still whistle?

I need car advice.. I’m not good with cars. The only things I know are how to change the oil, tires and the brake pads.

I just bought brand new brake pads and made sure I got the most expensive ones just because I know you get what you pay for and I drive kids around. I drive a 2011 Nissan.

I changed them a year ago but it sounded like the brakes were metal to metal and I could even hear it when I turned the wheel it sounded awful. I take my old brake pads off and they’re not even half way worn! I put the new ones on and drove around and the metal to metal sound is gone but there’s still that squealing whistling sound when I brake. It’s only when I come to a slow stop.

I’m just 😡😡😡😡😡😡😡 ugh! Why!

You all seem more skilled at life than I am and I’ve seen you all answer car questions before and I’m about to rip my hair out with how annoyed I am that I just spent time changing my brakes and I still hear a “WHEEEEEEE” when I come to a stop