Rationally, you are a human, making a mistake, that's what we do. We **** up, forgive ourselves, learn, grow, evolve, repeat.

There are serious physical and mental consequences for not forgiving yourself. (I have no idea what you did or how bad it was - if others were hurt, you make amends). But not fogiving yourself can lead to hating yourself, hating your life, hurting yourself, and so on, false beliefs about yourself that hinder your growth and healing, can really make you go crazy. If you want to be truly healthy, you need to know there's no room for hate in your body, and that at some point you must forgive yourself.

That's how I rationalize it, even if i've disgusted myself, forgiveness of self is something that must be done.

As for how I go about is through positive self talk like Dan is saying, or through meditation which becomes becomes metaphysical and spiritual for me where I smash false beliefs and so on.