I clicked Decline on my own store and now the cookie banner never shows. How do I see it again?

That's expected. Once a visitor clicks Accept or Decline, their choice is saved in that browser for one year and the banner stays hidden for them.

To see it again:

1. Open your store in a private or incognito window, or
2. Clear your store's cookies in your browser and reload the page.

You don't need to do this just to check your changes. The Preview box on Settings → Web Settings → Cookie Consent updates as you type, so you can check the wording there before you click Save settings.