We are creatures of habit and our life is dictated by what habits we are regularly allowing to control us. We get habits by reacting to external stimuli in repeated ways, so that our minds get used to responding to certain triggers in that way. The habit cycle works like this:
Some external stimuli triggers us.
We generate thoughts based on that trigger.
Our thoughts lead us to action.
This is where our power of choice comes in. The triggers are going to happen. External stimuli are out of our control. But we do have the power to choose our thoughts and our actions.
Here's an example:
I am driving in the parking lot and a guy cuts me off. I think "what a jerk!" and honk the horn at him so he can see me give him the finger. The other guy in turn feels pretty bad and gets angry too, and now both of us are in bad moods. So the trigger here was the guy cutting me off. My initial thought of "what a jerk!" is a normal response for most of us, but it's at this point that I can choose how to respond to that thought. I could get angry and demonstrate to the other guy my displeasure with his driving abilities OR I can take a new perspective. I can let it go and consider why he cut me off. Maybe he was in a rush to get home because his wife is pregnant, or maybe he just bought that new video game he really wants to play. Either way, no matter the importance of his reasoning, he probably isn't thinking about me when he cut me off. It is unlikely that he meant any personal offence towards me. So why should I take it personally?
Letting go of things such as this has a lot to do with forming better habits. If I continuously get mad at other drivers on the road, then I will get used to reacting this way and it will become a habit so that I always get mad at other drivers. But if I can let go of other people's bad driving and realize it has nothing to do with me, then I can form a habit of forgiveness. It all lies in our power of choice.