Conflict is just blahhhhhhh. But in close/deep relationships it will happen; you are two different people, raised differently and with different beliefs. Conflict tests your pride so hard; it's too easy to put all the blame on the other person and walk away thinking you did nothing wrong. The problem with this is that you will never learn how to have a successful relationship and you will continually hurt others.
My first piece of advice is to take the time to process the ACTUAL issue. Taking it on in the heat of the moment will lead you to say things you wouldn't normally say. While processing it try to get out of the mindset of how they hurt you and think about what you might have done to contribute to the issue.
PRAY before the "confrontation" and go into it with the intention of gaining peace and not to attack to get revenge. Make sure to talk in person and not over text.
In the midst of the confrontation focus on the single issue, try not to bring up what they did to you 5 years ago. If you can't solve the issue with them find someone wiser and older than you who can give you Godly advice on what to do next. Be careful though, this has to be someone who is going to tell you what you need to hear and not what you want to hear. It could easily become a pointless venting session that ends with you corrupting your friends judgement of that person.
Hopefully these few tips will help you work through any conflict you have with people. Just overall seek to have peace and seek to bring God honor. If you feel bitterness and hatred pray that God would remove it so that you could see clearly.
"Bear with each other and forgive one another if any of you has a grievance against someone. Forgive as the Lord forgave you." [Colossions 3:13]