Help! Messed up everything somehow

https://codex.www.ads-software.com/Moving_WordPress
https://codex.www.ads-software.com/Giving_WordPress_Its_Own_Directory

P.S. I hope you made a backup SQL before you moved anything.

Just for fun, what happens when you type in //www.mydomain.com/wordpress/wp-admin/ ?

You are expressing a concern without relating a lot of details. Having said that, here is the short answer assuming all unknowns are equal and common.

You stated that your blog is not installed at mydomain.com. It is installed at mydomain.com/wordpress. Unless you moved the files out of the sub folder, changing the address will result in problems. Log into whatever sql administration panel you use (phpmyadmin, if you are lucky), and change the address back to mydomain.com/wordpress and start over.

Here is a tutorial for changing your address using phpmyadmin.

https://www.tamba2.org.uk/wordpress/site-url/

…and additional reading that might point you in the correct direction.

https://codex.www.ads-software.com/Changing_The_Site_URL

….additionally, you may want to research the differences and requirements related to moving to or from, or setting up your blog in a sub-directory.

Best of luck.