i want to go back to mojave without deleting anything from my drive. i have a 2017 macbook with only a usb-c port.

Don't ever try this without a full backup.

You will really need to erase the drive.

It may look like a pain, but there is no other way.

Create a bootable installer on a thumb drive, and make sure you have an up to date Time Machine backup, a bootable clone of your drive or, preferably, both.

Boot from the installer, open Disk Utility, make sure that "Show All Devices" is checked, select the whole drive, and erase it.

Then quit Disk Utility, install Mojave, and, when asked, migrate the user accounts.

https://www.imore.com/how-downgrade-macos

You cannot just leave your data as it is.

You will need to export your Photos library contents and your music for a start before downgrading, as Catalina has reanalysed and reorganised them