In any relationship, there is nothing more valuable than a short-term memory. Grudges kill. Most indiscretions are forgivable, and learning to let it go is paramount to maintaining a healthy life. You can carry the baggage of past mistakes forever, or you can let it go and move on. A slight, a mistake, a misconception, a bad day, these things happen.
Sometimes, "sorry" has to be enough penance to pay.