while loop inside a trigger to loop through all the columns of table in sql

I have a trigger like below on user table to insert into the audit table with which column was updated and previous value:

ALTER TRIGGER [dbo].[trgAfterUpdate] ON [dbo].[tbl_User]
AFTER UPDATE
AS


    declare @fieldname varchar(128) ;
    declare @OldValue varchar(255);
    declare @CreateUser varchar(100) ;
    declare @User_Key int;

    select @CreateUser =i.user_name from deleted i; 
    SELECT @User_Key = i.user_key from inserted i;  

    if update(user_name)
      begin
          select @OldValue=j.user_name from deleted j;  
          set @fieldname = 'user_name';

            insert into tbl_Audit(user_key, field_name, previuos_Value, user_name)
            values(@User_Key ,@fieldname,@OldValue, @CreateUser);

      end

But my questions is I have like 100 fields on my table. I can't write 100 if conditions. And i need a suggestion how to use while loop in it, and how is it going to effect the performance.

Thanks