Forgiveness is the starting point, but overcoming bitterness can sometimes take longer. Ask yourself why is the cause of that bitterness - perhaps the effects of what your friend did (if she made you lose something - time, self-respect, trust in other people), perhaps you think she doesn’t know how much she hurt you, perhaps you don’t believe that she was sincere when she apologized to you. Either way, you are free to leave these feelings behind; since the past cannot be undone, you are causing yourself useless pain by dwelling on the past.
Christians have a great quote to meditate on: “Father, forgive them, for they don’t know what they are doing”. Jesus really meant it; if we think a bit, we’ll see that it applies literally to other people who hurt us. You don’t have to invent excuses for anyone so as to be able to forgive them; this is the reality, very few people hurt others with full intent and perfect clarity of mind. And even if this is the case, asking for forgiveness means that they are sorry, even when they can’t undo the harm and even when they don’t fully understand how much harm they have done.
Pope Francis says: “Are you angry with someone? Pray for that person. That is what Christian love is”. Praying for people who have wronged us can always help us to overcome our bitterness. If you’re not a Christian, the equivalent of praying for these people is wishing them good - wishing them to become better, to grow up, to derive their happiness from doing good to others. This, in turn, helps us to see them in a better light, as persons who are exactly as fallible as ourselves and exactly as capable to evolve as ourselves: this is the meaning of the well-known “how many times must I forgive my brother? I say to you, not seven times but seventy-seven times”.