Why can't I overcome my bitterness and anger?
Most of our bitterness and anger towards others is rooted in an inability to be profoundly amazed at Christ's love for us in our sin. If you are struggling with bitterness then it may be that the Lord is letting the very sin that is flowing from your inability to see Christ be the means by which you come to see him.
In other words, perhaps this season of rage, anger, and a fed-up "I'm out of here and don't want anything to do with you" spirit is where you have had to come in order to see the greatness of your sin as a forgiven and justified saint. And the Lord has done it so that you would be stunned at his grace in a deeper way than you've ever been stunned by the grace of God before. And now, out of that experience can flow grace towards others.
That's the only solution here. I don't doubt that another person is part of the problem. This is probably not just a one-way thing and your fault only. But the solution is not to fix the other person. The solution is to gain a heart that is overflowingly thankful for grace from Christ and that spills over with grace towards others.
What I'm trying to draw attention to is that maybe God has brought you to this point of feeling your guilt so that grace would taste sweeter than it ever has. We have to see our sin, but some of us have grown up in such goody-goody homes that we don't think we've ever done anything serious.
But unforgiveness is a hell-bent sin. The Bible says that if you do not forgive those who sin against you, God will not forgive you (Matthew 6:15). In other words, this is a mortal issue. An ongoing, unforgiving, bitter, and angry spirit will kill a person's heart, making them shipwreck their faith and prove that they never belonged to God. God is showing you how serious this sin is.
This means that now you have the potential of saying, "If he loves me still, and he forgives this, it's like forgiving the apostle Paul!" (It's like forgiving murderers, because the Bible says that if you hate your brother you've killed him [Matthew 5:21-22]). And then maybe the emotional transaction of forgiveness and justification would so overwhelm you that the resources that you do not now have for loving this other person would be given you out of that fresh, new experience of grace.
That's what I would pray. "Forgive one another, as God in Christ forgave you" is an unbelievably important word in Ephesians 4:32. You now have the potential of feeling forgiven by God for things that are mortally dangerous, which might open the door for greater grace towards this other person.