Hiding how far behind you are doesn’t slow down the clock. Tell someone while there’s still time to catch up.

Being behind and being alone with it are two different problems, and the second one usually does the damage. While you keep it quiet the deadline is still moving, and each day removes options that existed the day before. Early on, things can be rearranged: an extension, a smaller version, somebody explaining the part you missed. Late on, all that is left is the consequence.

Pick the person who can actually change something and tell them the real number today. Not a softened version of it, the real one. Ask what is still possible from here, because that question tends to produce more than you expect.