Jesus brings us up pretty short in the matter of forgiveness for He said, "If you forgive men for their sins, you heavenly Father will forgive you. But if you do not forgive men, then your Father will not forgive you your sins. (Matthew 6:14-15)
So, yes, it's a sin to hold a grudge. We are called to forgive even up to 70 times 7 times. (Matthew 18:21-23)
What to do about it Biblically? Recognize that grudge holding is sinful, confess it and repent. Then forgive the one who sinned against you. Realize that forgiveness is good for you, it keeps bitterness and anger from taking root within you. Ask yourself this: Is what this person did to me, worse than my own sin which Christ went to the cross for?
Hopes this helps!