Cheri - The General Membership Private Message area has a limit on messages that can be archived. As the other person suggested, start a new folder in your OWN PC memory - possibly in "My Documents" and copy and paste your text into your Word Processor and save it in that location for safe storage. Just before you save it, give it a title name (Save As) including date and topic so you can scroll through them and find what you are looking for later on.
If there are a few special ones, you can also create folders within your mailbox and move them to that folder. When you purge your inbox and outbox, those are not affected until you purge them as well.