Telling the truth when a lie would be easier is a skill. Practice it on small things.

Honesty under pressure is not a decision you make in the moment; it is whatever you have already been practising. Small lies are frictionless and they set the default, so that when something significant arrives you reach for what your hands already know. The training happens on things that do not seem to matter.

Notice the next small convenient untruth and say the accurate thing instead. The one about why you are late, or whether you read it. Doing that ten times makes the large one possible.