Decide what you want to say. If a long period of time has passed since your breakup, or if your breakup went particularly badly, your ex may have reservations about receiving a letter from you. Use this letter to speak your mind, because you may not get another chance if he decides not to respond. You may want to start by recapping where you are in life, and why you thought to reach out to him.

Write your letter from a sincere and genuine frame of mind. Being sincere is a sign that you have grown from your experiences and recognize the impact of how your actions affected the nature of your previous relationship, according to Aaron Lazare, professor of psychiatry at the University of Massachusetts Medical School, in his Psychology Today article "Go Ahead, Say You're Sorry". Lazare suggests that after a fight or breakup, genuinely expressing your thoughts and feelings indicates to your ex that what occurred during your relationship was felt mutually, and, given the chance, you would not take the same course of action in the future.

Offer an apology if appropriate. Breakups rarely go smoothly, and, in many cases, both parties involved may have felt hurt, slighted, or wronged during the breakup. While an apology may not instantly mend the wounds left by your breakup, it may play a crucial part in the forgiveness process, suggest Johanna Kirchhoff, Ulrich Wagner, and Micha Strack in "Peace and Conflict: Journal of Peace Psychology." The 2012 study indicates that of an apology's various components, conveying emotion and the actual act of saying, "I'm sorry", were the most crucial related to forgiveness.

Choose your wording wisely. How you say something often carries more weight than what you say. A statement such as "I know this letter is out of the blue, but if you're comfortable with it, I'd welcome hearing back from you when you have the time," carries a different connotation than, "I miss you so much, and don't know what I'll do if you don't write back." although both statements indicate you're thinking about your ex, and you're hoping he'll write back, the former statement carries an undertone of confidence and security, while the latter comes across as needy and desperate. The language you use can help to create healthy boundaries should you and your ex reconnect, or it can serve to reinforce that poor boundaries may have interfered with your relationship in the past.