I can't forgive myself. What do I do?

Say what you did, exactly, without softening it and without performing your guilt. Then do the two things guilt is for: repair what can be repaired, and change what can be changed. After that, refusing forgiveness is not humility — it is a way of staying the main character. If you would forgive another person for this, you are allowed to be another person too.

The word is selichah — pardon: something you are allowed to receive, which is harder than earning it.

From the sources “As far as the east is from the west, so far does He remove our transgressions from us.” — Psalm 103