The only entry in our One Day at a Time in Al-Anon (ODAT) book that deals with forgiveness says that we can only forgive ourselves for judging someone in the first place.
So I looked in the ODAT:
We are asked to forgive those who have injured us. Unless we have first judged and condemned them for what they did, there would be no reason for us to forgive them. Rather, we would have to forgive ourselves for judging.
I hadn't come at it from that angle, but had arrived at much the same place from a different direction. Anytime I've tried actively to forgive someone, the results would be temporary. Eventually, the resentment would return and continue to poison my life.
I had read somewhere else that forgiveness is not something we do for someone else. There's every chance that whoever harmed us doesn't give a rat's ass whether we forgive them or not. In some cases they may not even remember we exist. Forgiveness is something we do for ourselves, to unshackle ourselves from the pain of the past.
So how do I do forgiveness for myself then? The answer is, I don't. It just comes when things in my life are healing. Forgiveness is more something I accept than enact, when I reach a realization that I don't have to feel a particular way anymore. When resentments melt, forgiveness happens. Or maybe that's a matter of semantics. Maybe letting go of resentments is forgiving. But I can't just up and let go of a resentment. There's a whole process there of understanding them for what they are, revising my judgments, and loving myself through whatever it was.
The first step, as always, seems to be acceptance.