The US Login and the EU Login are two totally sepatate things. You cannot use one with the other. Which is why currently you need two game client folders, one patched for US, one patched for EU. If you patched your game to EU without making a copy of your game folder, you'll have to patch it back to be able to play on Ywain with your US account.
Assuming you are patched to EU then you need to use the same username and password combination as you use to log into
https://eu-accounts.eamythic.comIf you can login to the EU account management, try setting a new password from there. This should also change your game login password. Bear in mind that passwords are case sensitive.