Send a new text without all the history for that recipient
The history is just for you. Only the message in the box is sent when you send a message, not the entire history. The recipient keeps their own history of messages which is unrelated to your history.

This is not email where a reply to a message includes a quoted copy of the original — each message is self-contained and it's up to the receiver to keep a historical copy of messages, which is what your device is doing for you.