Prophet Muhammad (PBUH) said: “The first matter that the slave will be brought to account for on the Day of Judgement is the prayer. If it is sound, then the rest of his deeds will be sound. And if it is bad, then the rest of his deeds will be bad.
It can be hard to forgive and let go but it's important to remember that harboring that resentment and holding a grudge can hurt you even more. The word "forgive" really means to give something up for yourself and not for them.