In loving relationships, there are several important things to know. One of these is that the key to a successful relationship is being able to apologize.
The more intimately you are involved with another person, the more difficult it becomes to say Im sorry.
You are aware of her faults and she is aware of yours. It is very easy in the heat of battle to hurt your partner in a very sensitive place. Your partner trusted you by forming a relationship and letting herself be vulnerable.
When you say, Im sorry, it must be done with the sensitivity to understand the line that was crossed.