Like any addiction, a grudge will wear a hole into you. Unlike many addictions, a grudge doesn't involve putting a substance into your body.
Instead it survives by getting you to focus only on the dark corners of your mind. Holding a grudge means that you are focused, not on those things that will make your life better, but on the person you're holding your grudge against.
The thing is that holding a grudge doesn't really affect the other person. Your hate and your focus on that other person's life really just affects you.
It puts you into a hole and into a life you never wanted to be in in the first place. All you're doing by holding a grudge is allowing the other person to grow faster than you.
How do you get over a grudge? It's not necessarily forgiveness, but rather just letting it go. The key is in taking your attention off of the other person and onto making your life better. Work on your fitness, your career, your business, and your positive relationships.