How do I change my cPanel username?

I am confused and feel like I have three different password and username combos because one combo may work for AMP but not cpanel, then it may work for cpanel but not to login in to support center.

My cpanel username prompt says it is happym10, however I want it to be [email protected]. When I change it it tells me my password is incorrect and so the password I used for AMP isn't correct. I had a very hard time getting my webmail set up which I seemed to do last night. The reason I had difficulty is because when I tried to run the webmail prompt it said Warning happym10 is what you are logged in as and may not be correct. so i followed the directions and changed it to [email protected] and changed my password. Now that same username ([email protected]) and password will not work for cpanel. I want one username and password to work for all of these because it is too confusing to try and remember the different combos. Thanks!