"I, the Lord, will forgive whom I will forgive, but of you it is required to forgive all men." D&C 64:10
"Behold, he who has repented of his sins, the same is forgiven, and I, the Lord, remember them no more. By this ye may know if a man repenteth of his sins—behold, he will confess them and forsake them." D&C 58:42-43
Forgiveness is not easy but it is a commandment given to us from our Father in Heaven. As we forgive others the things they have done to "wrong" us the lord promises that we can also be forgiven if we repent of the sins we have committed.
We are commanded to forgive everyone, even if they may have deeply hurt us, but God will forgive those whom he will forgive. He will forgive only those who desire to be forgiven. If we forgive then we are able to let go of that bitterness we are holding inside which keeps us from being happy. I think this video give a great example of this.