Forgiveness is a Gift to Yourself
Happiness takes practice. Seriously. It's not just something that happens, at least not most of the time.
Forgiveness doesn't come all at once, especially not for big hurts. Have you given yourself a chance to appropriately grieve or get past this issue? If you haven't, then you'll want to, because skipping the grieving part means that the issue will probably resurface later on, perhaps when you least expect it, just like the Wack-a-Mole carnival game. Healing can't be rushed....remember Elizabeth Kubler-Ross' 5 stages of grief: Denial, Anger, Bargaining, Depression and Acceptance. You'll note that "acceptance" is #5, not #1. It takes time to get to the end of it.
But what about when an appropriate amount of time has passed? Yet you're still wrestling with the issue-maybe your cousin's comment about your weight last Thanksgiving, the time you got pantsed on the playground in elementary school, or healing from the disappointment of a marriage that ended in divorce. Obviously, each of these three examples will have very different "get over it" time frames, but most people should be able to recover in time.
At some point you simply have to make a choice:
Given all the terrible things that happen in this world: war, famine, disease, crime....why am I still choosing to dwell on this?
Given that this has happened, and I have to live with that, what's the best I can do with that reality?
Could it be that the negative thing(s) which have happened to me do not have to color my entire life?
We become what we practice---what do I want to become?
The Stanford Forgiveness Project found that, basically, anyone who complains a lot is going to be unhappy, irrespective of what the suffering actually is. The issue is not the problem we've experienced, but what we choose to do with it.