You could try using includes to make code maintenance easier:
in .ASP your index page could look something like this:
<!-- #include virtual ="head.inc" -->
<!-- end head.inc -->
<!-- #include virtual ="nav.inc" -->
<!-- end nav.inc -->
<!-- #include virtual ="main-content.inc" -->
<!-- end main-content.inc -->
<!-- #include virtual ="sidebar.inc" -->
<!-- end sidebar.inc -->
<!-- #include virtual ="footer.inc" -->
<!-- end footer.inc -->
or with .PHP
<?php include("head.php"); ?>
<?php include("nav.php"); ?>
This way you are only editing the the seperate include files.
So instead of an 'insert content here' comment, you could just use an include call right in your html page, like so:
<?php include("main-content.php"); ?>
Hope all that makes sense
Actually, this is quite an interesting topic: a text editor that allows you to toggle the visibility of code blocks would be a great idea......I wonder if there is one?
Other than that, I'd give a more advanced text editor, like notepad plus mentioned above, or, my favourite: SciTE
which at least colour-code different types of code.