In "Not 'Just' Friends" read the chapter about Forgiveness and Moving Forward (p. 315).
Bitterness in a divorce is unresolved pain. So.. how do you ease and get rid of the pain, especially when everything seems so unfair? Well.. the concept the author, Shirley Glass, puts forward is to 'send blessings'. Whenever the object of your pain (we'll call it 'spouse') does something that makes you angry, hurts you, gets you reeling thinking about how unfair this all is, you send them a 'blessing'.. something positive.
When I read this I thought that had to be insane. However, I didn't want to lead an angry bitter life, where my emotions were still focused on spouse negatively, with me giving away energy that should go toward me and my family.
The first time I tried it, in my mind I sent him a herd of camels. I imagined with glee the mess such a gift would make in the parking lot outside of his domicile. I promised myself I'd be more charitable in the future.
So.. whenever I got wound up and pissed, I'd send him a blessing, good wishes. Initially it caught in my throat. The more I could send, the better I'd feel. Good health, happiness, long life.. were but a few. Each time it helped me let go of muck.
Someone was telling me that in AA, they tell you to say a prayer for the person who wronged you. It doesn't have to be a perfect prayer forgiving that person... just a prayer. You can say, "God, please look out for 'the person who wronged me' even though there's no f$cking way that should ever happen." That way allows you to express the anger.
However, the blessing thing and letting go of the first example gradually helped me feel better. When spouse's actions would flip me out, the blessings helped me get centered again, faster. I'd recover faster.
You do it for you... forgiveness is a gift you give yourself. Your energy goes toward positive aspects of your life.. Otherwise you're never able to let go of what hurt... in fact, you end up being the one to stab yourself.