My favourite definition of forgiveness is by K Bradford Brown, creator of the More to Life programme (highly recommended);
"The absolute refusal to hold ill will against someone for something they did or didn't do."
The foundation of forgiveness is the ability to see the meanings you have made about the circumstances and then to identify what holding onto the resentment is "costing" you. There are often "good reasons" for wanting to hold onto the resentment which many people don't recognize in themselves. One common reason is sometimes it seems easier to hold onto the resentment than to experience the pain of the loss, betrayal or difficult circumstances.
When I work with my clients on letting go of resentment and hatred, I use a blend of several processes I've learned during the past 25 years, and have supported the "release" of my clients from the burden of carrying the unresolved trauma buried beneath this.
Forgiveness IS a choice. This doesn't necessarily mean that it is easy and supporting others through to complete forgiveness is a particular passion of mine. I have helped many people come to a place of peace with their past (sometimes from very traumatic and difficult experiences).
When you truly resolve your past (rather than try to ignore it or gloss over it), you become free in the moment, fully available to what is present in your life. When you are fully present, your experience is naturally richer and more vibrant, and you can begin to sense the infinite possibilities life is inviting you to enjoy...
I also have a theory (unproven as yet) that the less "drag" there is from the past, the healthier our cellular vitality is... So if vanity is in any way a motivational tool for you - having less resentment could make you look more beautiful as well as feeling it!
See also my blog on Best Practices: Release The Past