Dear Emma,
This can be a difficult thing to do by human standards. Our fallen nature tends to agree with the Mosaic law, "an eye for an eye and a tooth for a tooth". One thing that will help you is for you to remember how much God has forgiven you. Remember the unmerciful servant in Matthew 18 who after having been forgiven all his debts went and grabbed another person who owed him a small amount of money and threw him in jail. His master heard it and grabbed him and he was thrown in jail until he had paid it all.
Another cure? The LORD's prayer; "forgive us our debts as we forgive them that tresspass against us" "And forgive us our debts, as we forgive our debtors." (Matthew 6:12).
In the book FORGIVENESS MADE EASY, Bishop Dag Heward-Mills advised that you insert the name of a person that you are having the problem about forgiveness in the place of "debtors" to bring into focus the reality of the situation. May these few words be of help to you. You can purchase a digital version or even a hard copy of this book, read it for yourself and may find yourself giving copies to many other people.