It’s going to happen. In fact, I know for a fact it happened before.
You screwed up.
You didn’t show up.
You outright lied (to yourself or others, both are bad).
You said you were going to do something and you didn’t do it.
You said you weren’t going to do something and you did it.
Whatever. It is over now. You screwed up. We all do it. What is important is what you do afterward.
That is what really, really counts.
Let’s get to fixing up the issue.
Did your screw up hurt someone?
Yes? Was it you or someone else? If you, apologize to yourself. Yes, go to the mirror and do it. Don’t beat yourself up. Be gentle. Forgive yourself, figure out how to move past it and do it.
Someone else? Apologize sincerely for the screw up, make amends if possible, let them be angry, and forgive yourself.
Is your screw up irreparable?
What can you do, if anything, to at least fix it? If nothing, apologies to the appropriate people/organizations sincerely, move on and don’t do it again.
If not irreparable, fix the problem. NO MATTER WHAT IT TAKES. You screwed up, you now have to face your actions (or inactions) have caused something, and it is yours to fix. Fix it. Forgive yourself and move on.
Do NOT beat yourself up endlessly.
Do NOT do nothing.
We all screw up; it is what you do afterward that matters.