"I'm sorry."
A phrase probably as misused as "I love you." There comes a point in every relationship where just saying those words is not enough anymore, simply because they are only words being said with no feeling or sincerity behind them. This is especially problematic today with the amount of technology that we have. Just as it is said it is easier to say mean things to a person behind a phone or computer, it is also easier to apologize and have absolutely no actual regret or sympathy for the situation. I have personally been in disagreements with friends who automatically begin to profusely apologize just so they won't have to admit their wrongdoings and actually talk about the situation. These meaningless apologies only lead me to ask them the question, "What is it that you're sorry for?" You should not be surprised to find out that too commonly they can not even answer this question regarding their own apology.
I'm sure I've done this a few times myself just to move on with the situation and make someone happy, but I have made it a point in my life to see every side of each disagreement or argument that I am either a part of or being asked advice for, and to make sure that if I am apologizing, that I know what I'm sorry for, that I am truly the one in the wrong and that I don't do the same things again to lead to future arguments.
One of the most important things I have ever learned is from a quote I found on Pinterest: "The best apology is changed behavior," and that could not possibly be more true. And again, this is something that must not only be said but done, as we hear very often actions speak louder than words. With this change comes the responsibility of making it a lasting thing. Change is not for a day or a couple months; it's a lifestyle choice, and the value that you put on the relationship that you are changing for should determine the longevity of this change. If you are going to keep doing the same thing repeatedly that you apologized for, then there is no sense in apologizing at all. Prove that you are sorry and change your actions. If you are unable to do this, then walk away either forever or temporarily until you are ready to make a change. There is no good in being a toxic part of someone's life.
Besides making a change and probably more importantly to some, if you're going to apologize, mean it. Don't just say it. Evaluate the situation. Don't apologize when you're angry; look at it from the other person's perspective and really try to understand how they feel. If the person on the other end doesn't forgive you, give them time. If you are close, the other person will be able to sense when you are genuinely sorry rather than when you're just saying sorry. Since we are so used to hiding behind a phone, you may find this is a difficult thing to do, but face-to-face apologies are so much more meaningful than over a text or Facebook message. This way, your emotions can truly be seen by the other person.
The bottom line is, "I'm sorry" isn't always going to be enough. You need to take full responsibility, work to regain trust, think about the future, leave nothing unsaid and ask for forgiveness. Something for both the person apologizing and the person on the other end to keep in mind: when someone truly cares about you, they will make an effort, not an excuse.