If anyone has ever hurt you, I trust you'll agree when I say that what Paul commands his readers to do in this passage is impossible without the Holy Spirit working in us.
Remember that forgiveness is: (1) a decision made in a moment in time, (2) a process whereby our emotions are aligned with that decision (often very gradually), and (3) a work perfected and completed by the Spirit's power in us.
Let us consider the amazing forgiveness God offered us through Christ's death and resurrection and thus be moved to forgive others, though it may be a length and uncomfortable process.