Thanks. This might not be the most direct way to do this but for anyone who got stuck like me, here’s the detailed explanation. I had set this up so long ago that I had forgotten how all the parts worked together so the summary help comments above didn’t provide enough detail for me to immediately figure out how to fix this. This should be enough for anyone who is still trying to work this out.
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1. Go into the wp-content\plugins directory and rename “theme-my-login” directory to a different name. This effectively disables the plugin.
2. Go to your regular login page. At this point, with “Theme My Login” disabled, you should see the default WP login form.
3. Once logged in as admin, from your list of pages, find your custom login page and open it. Change the shortcode showing from [theme-my-login-page] into [theme-my-login]. Save the change.
4. Go back into wp-content\plugins and rename the “theme-my-login” directory back to its original name.
5. Go to “Plugins” in the Admin section and re-enable the “Theme My Login” plugin.
6. Log out of the Admin section and go back to your login page. You should now see your “Theme My Login” based login form.