I believe Wunjo would be best, due to its relation with happiness. Whenever you are dealing with problems of the self, my first place to go would be solar solutions. As a secondary solution, earth-based energies can help one be realistic about this and understand that is was in the past.
If the mistake is really bad, you could even do a blessing towards any hurt individuals or something, if this applies. Ansuz could be useful for dealing with a specific event, but if you are prone to self-hatred, or excess criticism, then working with Wunjo or Sowilo would be closer to the root of the problem.
Regarding whether this is waxing or waning, I think it could go either way, as long as the affirmation supports it.