Some things are hard to forgive. They may be major indiscretions or minor irritations. It’s hard to say why some things are just so difficult to let go of, but be aware that just because you have already forgiven someone for that thing doesn’t mean that it’s gone. If you still carry resentment or hurt, then it looks like you took your forgiveness back.
Peter asked Jesus, Lord, how often shall my brother sin against me and I forgive him? Up to seven times? Jesus said to him, I do not say to you up to seven times, but up to seventy times seven. (Mat 18:21) In other words, you need to forgive for as long as there is need to. And it can be irritating. You may say to God, but I already forgave him for that. God will say to you, then forgive him again, you didn’t let go of it.
You’re supposed to be a lamp on a hill. Any unforgiveness you hang on to is like a film that covers that lamp. It’s unattractive and useless as a lamp. God knows that it’s hard, so don’t beat yourself up if it means that you have to forgive the same offense several times, but you need to do it until you can let it go. Start to bless the person who hurt you. Pray for them and the forgiveness will come easier. Forgiving someone again, even if it’s old and forgotten by everyone else, will help you grow in favor with God. It doesn’t matter how many times you have to do it. If it still hurts, then forgive again. Every time you do, your lamp burns brighter.