The good news is that if you are able to acknowledge you are holding a grudge, you're halfway to releasing it. To fully let it go, try a forgiveness meditation. Each night in a quiet room with eyes closed, repeat this as often as you need, either out loud or to yourself:
'I forgive those who have wronged me, I forgive myself. I let go of all anger, resentment, and pain. I welcome an abundance of love, peace, and joy. Thank you, mind. Thank you, body.'
Do this every night until you feel you no longer need it. Even if you don't reach 100 percent forgiveness, just repeating the words helps. Research overwhelmingly supports that meditation makes it possible for you to improve your mental and physical state.