Speak and act as those who are going to be judged by the law that gives freedom, because judgment without mercy will be shown to anyone who has not been merciful. Mercy triumphs over judgement.
James 2:12-13
I think this a difficult topic because we would like grace to be a complete pass with no strings attached to it. But, James is clearly saying that if you are not merciful, there is no mercy for you.
Jesus had taught the disciples in Matthew 6:14-15, "For if you forgive men when they sin against you, your heavenly Father will also forgive you. But if you do not forgive men their sins, your Father will not forgive your sins."
When you have unresolved issues of forgiveness in your heart they are attached to your inner being, whether you acknowledge them or not. Sometimes we like to just push things to the side and try not to notice them or deal with them, like a pile of bills on your desk that you refuse to open. Just because I refuse to deal with does not negate their presence or affect on you.
Think about this; the model prayer that Christ taught his disciples has a postscript that flat out states that if we won't forgive someone who sins against us, we won't be forgiven. Because Christ first forgave us (See Colossians 3:13). I know that some people can be "impossible" to deal with, but, we are not being asked to spend Thanksgiving with someone who has sinned against us, we are being told to forgive them. Whatever God does after we forgive them is up to God.
