This is an interesting question because if you want to rewrite the content, GPT will rewrite the content (which could include trimming the original text). With that said, have you tried to prompt GPT before adding the text you want to be rewritten?

For example, a prompt I've used: "Without starting, I'm going to post something below for you to rewrite. I want you to keep as much of the original wording and sentence structure as possible. Make only minor adjustments to improve clarity, grammar, and flow. Do you understand?"

Then GPT will respond with something like Yes and give you a quick response of understanding.

Next, you can say something like, "Keeping the above in mind, please rewrite the following [insert text]."

There might be other things to try, but this is my first suggestion. If you want other ideas, you can also try something like: "Rewrite the below text with slight changes to improve grammar and readability. Keep all key points and phrases unchanged unless necessary for clarity."

Hope this helps!