If you just installed the latest preview of SSMS 18.0 you might have realized that the Database Diagrams are gone. The feature is deprecated.
Download the preview here:
https://docs.microsoft.com/en-us/sql/ssms/download-sql-server-management-studio-ssms
Scroll down to the SSMS 18.0 (Preview) and try it out, there is also improvements.
So, what to do? Remember there is a bunch of INFORMATION_SCHEMA-views you can use.
This is an example how you can do it, the example is not perfect but help you out a bit.
As Always, I do not take any responsibility for any code you might find on my blog. I write queries for fun and business. Small snippets might end up here.