How do I split my current Apple ID into two accounts?

Why do you want to create a new Apple ID?

With Family sharing, other family members only have access to what you share. There is no access to your iCloud content, mail, messages, photos (except shared ones) etc...

If they are having access to everything, then you are using Family sharing wrong.

Each member of the Family Sharing group must have their own Apple ID logged in to their own devices. If you have your Apple ID logged in to all devices, then that's a mistake.

FamilySharing works across different Apple IDs. There is no reason to use your own on all devices. In fact if you have it logged in to all devices, then you are entirely defeating the purpose of Family sharing. Other members of the Family Group can access iTunes and App Store purchases over Family sharing just fine.

See Family Sharing: link-> Set up Family Sharing - Apple Support

and here link-> Share App Store, iTunes Store, and Apple Books purchases with Family Sharing - Apple Support

I would suggest you get Family Sharing set up correctly. It will be the best and easiest choice in the long run.

You can't split an Apple ID. You would have to create a new Apple ID.

Apple ID - create a new.      

Apple ID – Create         

There may be some data loss, but you can minimize it. If syncing photos with iCloud, ensure Optimization is turned off in Settings/Your Name/iCloud/Photos and/or in Photos/Preferences/iCloud. Then make sure full resolution photos are on the device/computer. When signing out of iCloud, if asked whether you want to retain data, choose yes. Messages conversations will be lost if they are addressed to the ID. See the article below for other places that the old ID should be removed. See Winston Churchill's response in the below discussion. Also set up Family Sharing between the IDs to keep access to your apps/purchases. 

How to switch Apple IDs on your iPhone or iPad.      

How to transfer all files from one iCloud account to another.        

What to do after you change your Apple ID or password - Apple Support.     

That should correct the problem.

Yes. You don't even need to change password for your account, since it will no longer be on their devices, they would have no way of accessing it.