A: Yes, in Matthew 6:15 Jesus told us, "But if you do not forgive others their sins, your Father will not forgive your sins." I'm glad your preacher gave you that verse. It's not one that most Christians memorize. Nevertheless it is a sobering truth about the life of a Christian.
You've hit on an important point - and that is that sometimes the forgiveness that's required is beyond human ability. In such cases, it will take a supernatural power to bring about that forgiveness in you. So you get on your knees and you confess the sin of unforgiveness. Agree with God that the forgiveness is appropriate and necessary. But confess to Him that you are not able to get to that forgiveness on your own. Ask God to help you. Seriously. Ask God to help you forgive that other person for whatever it was they did that caused such a serious wound in you.
The thing about forgiveness is that it is not a feeling. Nor is it tied to our feelings. So you don't, for example, forgive someone when the pain is gone. In fact, you could forgive someone when the pain is at its greatest. That's because forgiveness is something that we choose ... and not something that we feel. And if the pain is so great that you're unable to make that choice on your own, then confess that and ask God to help you. It's a prayer request He'd love to answer!