Don’t worry, we all have to start somewhere!
Once you’ve split the closed section of the highway, it might be worth using a lifecycle prefix, particularly if it still appears to be open in aerial imagery.
For example, if the road surface has been removed, you might change the highway tag to razed:highway. If everything is intact, it’s been fenced off and could (in principle) be reopened, you might use disused:highway or abandoned:highway
See Lifecycle prefix - OpenStreetMap Wiki for more information.