Summarize:

A break-up should be a clean break with no room for ambiguity. You should be honest from the get-go you want to end the relationship. The letter's intent should be obvious from the opening line. Easing someone into a break-up seems less cruel, but it actually comes off as a mixed message. It can easily be read as uncertainty, making your partner think there's a chance you might change your mind. Your opening line should be concise. Begin with a simple sentence that communicates the letter's intent. Something like, "I have come to the difficult decision to end this relationship" or something along those lines works. This comes off as certain, leaving no room for confusion. Breakups can be difficult and cause a lot of feelings of failure and anxiety for both parties. Open communication is important as it removes the stress of not knowing. Your partner deserves to understand why the relationship ended, so providing specifics as to why you feel things aren't working out is vital to a break-up letter. Information-sharing is an important aspect of any relationship. In a breakup, sharing the reasons you feel the relationship should end conveys you respect your partner and want him or her to have closure. The idea of a winner (you, as you're leaving) and a loser (your partner, as he's being left) are diminished if you're honest. The blame is not on one individual but rather the incompatibility between the two of you. Even if your partner did something that made you unhappy, avoid name-calling and blaming. Simply state the specific behavior that resulted in incompatibility. Instead of saying, "Your friendship with your ex is damaging to our relationship" try to state it in a way that focuses on your personal feelings rather than applying objective facts. For example, "The fact you and Jane are still so close makes me feel it would be difficult for me to trust you longterm." Specific behaviors that were damaging to your relationship are important for your ex to know. He or she can use this information to improve his romantic relationships in the future. If you phrase things in a non-confrontational way, it may be helpful for him of her in the future to know why a past relationship ended. Remember, it's not always something the other person is doing. Sometimes, incompatibility causes a relationship to end. If this is the case, say so. This is often easier to accept and your ex will not end up feeling bad or guilty about past mistakes. Say something like, "I feel that we ultimately want to end up in different locations and as I'm getting older, I want to find someone I'm compatible with in the longterm." Remember, as you're not face-to-face the non-verbal cues that can be reassuring are absent. Kindness is important in a break-up letter. You want your partner to understand that, even though the relationship has ended, they still matter. While you need to be firm that you're ending the relationship, there's no need to be spiteful or aggressive while doing so. Even if you do have feelings of resentment, expressing yourself is an aggressive way could lead your ex to retaliate with his own letter or an angry phone call. End the letter by saying something positive, like "I valued the time we spent together and I hope you find someone you work better with in the future." The absence of non-verbal cues can make you come off as cold, so you want your ex to feel that you do care and that he or she is valued even though the relationship is over. Another reasons kindness is important is that letters bind you to your words. In anger or sadness, we often say things we do not truly mean and you do not want you or your ex to carry around physical evidence of a regrettable phrasing or tone.
Be honest about the letter's intent. Explain specifics. Be kind.