I believe these two words go together. You become bitter because of the way you were treated. Soon you are unwilling to forgive the person, because the extreme bitterness you feel won’t budge. So how do you get rid of the un-forgiveness, and come to the place where you can truly forgive? It is by grace that we are saved, and we did nothing to make this happen. God was involved. He can be involved in your forgiving.
Set aside a block of time, and meet with God by yourself. Tell him about who what, where, and when this happened to you. He knows, but refresh Him about this. Ask him to take away the negative feelings, and the bitterness you feel. Truly repent of the bitterness, and then give it to God. You can rebuke it off you in Jesus name at this point. I believe it leaves when we are convinced that it must go in Jesus name. Forgive, if it takes a year to get over what you have been through. Forgive, because God can’t work on the persons who were mean,until you forgive them. It’s hard, but do this, and you will feel the liberty you now have in Christ. With bitterness, and un-forgiveness gone, you are free to stand in the Lord, in His Power.