I never really understood the concept of forgiveness while growing up, maybe because the idea of sin and forgiveness seemed to increase my sense of guilt rather than allay it. Alot of other spiritual paths seemed to ignore it altogether. Perhaps the most interesting and thorough explanations I've read about forgiveness comes from ACIM. Just to cite one sentence: "Forgiveness looks quietly, and judges not." The world's (ego's) idea of forgiveness is to judge first and then somehow try to be the better person by letting the other off the hook. ACIM resolves this by it's claim (as Vivamis has said in her post) that there is "nothing to forgive" because we haven't changed God or Heaven one iota by imagining that we could be seperate from our Creator.

In our "state" (our perceived reality as separate beings) however, forgiveness takes practice. And I agree that whatever you give to others, you give to yourself (and vice versa). If there's one thing I've learned in this life, it's that you only judge in others what you judge in yourself. So, yeah, self-forgiveness is necessary, at least until we realize we need not judge at all.