John Howard asked for some practical steps to forgiveness. It's a great question. I'm going to try to tackle it here. I'd love to hear your feedback and your suggestions.
1. If you are a child of God, the first step is to realize that you must (not should or ought) forgive. A little conviction of sin--yes, I said sin--is in order here. I know; it's what drove me to forgive. Take some time to meditate on the following Bible passages. (Exodus 20:12; Ephesians 6:1-3; Matthew 5:7; Matthew 5:22-24; Matthew 18:21-35) Then pray. Confess to God that you are harboring unforgiveness and that you really don't feel like forgiving. Ask Him to give you the grace to do so anyway, in spite of your feelings. If it helps, ask a friend to pray with you and hold you accountable for your obedience.
2. Reach out. OK, you don't feel like it; but do it anyway. Forgiveness is a choice, not a feeling. Pick up the phone, write a note, send flowers, drive over and visit. Just do it. It's going to feel awkward and weird, but it will break the ice. Repeat as necessary. It will get better, I promise.
3. Accept your parent for who they are. I don't mean sweep everything under the rug, but I learned this extremely helpful tip from Dr. Kevin Leman. He pointed out that daughters have a tendency to hold their fathers accountable to the daughter's ideal of a father--what he should be. Trouble is, he isn't that guy, and he's never going to be. She's always going to be frustrated and disappointed until she stops holding him accountable for the ideal and starts figuring out who he really is and learning to love that guy. Oo, this is a hard one. It's hard to let go of our expectations and our right to be disappointed. But just try it. Try being thankful for the good things your parent offers. It may not be much, but be thankful for what you've got.
4. Confront sin biblically. The advice here has to differ based on whether your parent is a believer or not. If you have an unbelieving parent, well, what more do you expect than a pile of icky sin? What's so surprising about that? Approach them lovingly, but recognize that without the same God-ward focus that you have, they may not even see their sin as morally wrong. Pray for them, share the Gospel with them, and love them with all you've got. It's the Holy Spirit's job to convict them of sin. Now, if you have a believing parent who is actively engaged in a sinful lifestyle (maybe indulging in an adulterous affair, or living with a new partner outside of marriage), you should follow the biblical protocol laid out in Matthew 18:15-17. Your goal is not to punish, remember, but to restore. And Jesus says that if the person professing to be a believer does not respond to discipline, you are to "treat him as you would a pagan or a tax collector." What does that mean? 1 Corinthians 5 is helpful here, because Paul makes it very clear that Christians and non-Christians should be held to different standards. But before you get too harsh with the expelling stuff, remember what Paul says in 2 Corinthians 2:5-11.
5. Recognize that forgiving doesn't mean everything will be just the same as it used to be. When a relationship is broken, scars are left. I think when people want us to "sweep it under the rug" or "get over it," they want to pretend that nothing bad has happened. They want everything to be the way it once was, before the hurt occurred. That isn't going to happen. It just isn't. So face that and figure out how it can be now. You've got to forge a new relationship, one that grows from the ashes of the old broken one. If trust has been broken, you might be wise to wait for the other person to earn your trust again. You don't need to be naive to forgive.