Verse 31 says "Let all bitterness and wrath and anger and clamor and slander be put away from you, along with malice". and verse 32 says "Be kind to one another, tenderhearted, forgiving one another, as God in Christ forgave you".
Anger is an emotion and unforgiveness is a sin and a choice. We may feel anger towards someone who hurt us which in turn can make us bitter and it can be a struggle to forgive them. But God tells us that we need to forgive - "as God in Christ forgave us".
When someone hurts us, it's so easy to stay angry and it's easy to want them to hurt like you, but usually they're over it and it's us that isn't. And it can be difficult to let go of those hurt feelings and forgive that person and move on. I know I have struggled with that and it's been hard and it's taken a long time, but with God's grace and some close Christian friends, I've been able to forgive them and I know that pleases God. It was a big heavy load I turned over to Him and He took it. Thank you, Lord God.
If we don't forgive those that have or are hurting us, we should read and reread this Scripture, turn our thoughts over to Him and ask Him to help us and to give us the strength to let go and be forgiving.