How can I customize the default login page in SharePoint 2013 with FBA authentication, but will not create a new login page, I want to modify the page that already exists.
The deafult login page is located here: C:\Program Files\Common Files\Microsoft Shared\Web Server Extensions\15\TEMPLATE\IDENTITYMODEL\LOGIN\default.aspx. You can modify that in Visual Studio. I know you said that you didn't want to create a new one but you could make a copy of the deafult one, add that to a new project and make your changes to the copy. That way you will preserve the default login page if you need to revert back.