Most of the basic phones don't store more than 100-200 text messages and will delete the oldest automatically if the limit is reached. Lock the ones you really need to save and clear your inbox and sent messages regularly.
The user manual says it can save close to 800 - check the "auto save sent messages"; if you are using threaded messaging (View by contact vs view by date) and it is not saving the sent messages, it may be deleting the threads as well.
From the user guide, page 77:
2. Touch Messaging .
3. Touch More.
4. Touch Settings to access following settings:
Message View/ Language/ Text
Entry Settings/Auto Erase Inbox/ TXT Auto
View/ TXT Auto Scroll/ Media
Message Auto Receive/
Voicemail #/ Callback #/
Signature/ Delivery Receipt/
Message Font Size/ Public
Alert/ TXT-Media Msg
Notification/ Reply Option
NOTE Please note that menu instructions
in this guide may vary if changing
the Message View option.