Who to tell, and how quickly

Tell somebody early. A personal tutor, the module leader, or the student support office — and early means before the deadline or before the exam, not after the result arrives. Almost every mechanism an institution has for helping you requires that it knew in time to use one.

The instinct runs the other way: to go quiet, because saying it aloud makes it real. That instinct costs people entire years. Staff deal with this constantly, they are not shocked by it, and the conversation is far shorter and far less awful than the version you are currently imagining.