As far as I know, you have the correct password, so it seems you changed the MySQL password as well, and should have put this in your password managerl…
There is a way to create a second user, then give this user access rights etc. This Login into Emonpi - #6 by pb66 gives the details, though it concentrates on a lost password, it might help with a lost user name.
I don’t know MySQL well enough to know whether this will work.