Have you changed the database prefix to "myusername" or is that what it actually says?
I am guessing you have changed it but it doesn't hurt to ask!
Go into phpmyAdmin and go into the database (either select the DB if you have more than 1 DB or it will automatically go into that DB if it is the only one) and then look down the list of tables on the left hand side - does wp_categories exist?
If you trust a stranger on a forum enough you can send me your hosting control panel log-in details to
sales@twdhosting.co.uk and I will take a look at the DB for you.