Have you cleaned up after yourself--? Apologized, made restitution? Gotten therapy? Pledged never to do "it" again? Etc??????? If so, stop worrying about forgiving yourself and start giving yourself reasons to think of yourself as a decent person.

Do good now. Whenever you get down on yourself over your past behavior, do good now. Even if it is to smile at a stranger, give a buck to a person who needs it, volunteering at a charity thrift store, visiting a nursing home resident out of the blue...... . Do so much good that even you can see it.

Use your guilt, don't let it use you.