True maturity is being able to admit that you’ve made a mistake.
If I apologize for something, that’s my way of acknowledging that I’ve done something wrong and that I feel sorry because of it. If I apologize and you continue to hold the past against me, I’m not the one being immature- that would be you. You can live with all the hate in your heart if you’re unwilling to forgive, but I’m not going to allow that to affect me. Deal with your problems in whatever way you deem necessary, just leave me out of it.