Flash works in the user's browser, and needs to be coupled with server-side programming (and perhaps a secure server) for any kind of secure application.
At a simple-t-implement level, you could continue with your asp approach, and when logged in allow access to a directory or directory structure secured with .htaccess, where your Flash pages could rest.
If you want to put a Flash visual front end talk to your asp pages, then you'll need to get to work on Flash forms. If programming is all new to you and you need this in a business timescale, it may pay you to commission a developer to do what you need.
Otherwise, a few tutorials that might help you get started: