Hello bethfy,
Just to confirm, do you experience the same issue when starting your Mac up from safe mode? "Safe mode can help you to determine whether an issue is caused by software that loads as your Mac starts up."
Please check out this link, which explains how to use safe mode, and will guide you through that process: Use safe mode on your Mac
Before using safe mode
If possible, update macOS and your apps before using safe mode. This is more likely than any other step to resolve the kind of software issue that safe mode can help identify. If necessary, you can start up in safe mode and then update your software.
* Install the latest macOS updates.
* Install the latest updates for your third-party apps. If you got the app from the App Store, click the Updates tab in the App Store to check for updates. Or get the updates directly from the app developer or their website.
To be sure that none of your connected peripherals and accessories are at fault, test again after unplugging them from your Mac, including printers, drives, USB hubs, and other nonessential devices.
Please also check out the following link, which can help if your Mac stops or delays while starting up: If your Mac stops or delays while starting up
Thanks!