To effectively tell your boyfriend you want to break up because he cheated, it's important to be prepared for his response. It's possible he could become upset, violent or difficult to deal with when you have the discussion. Or, he may try to convince you of the reasons you should stay together. Do your best to leave the details of the cheating episode out of your discussion and focus on why you can no longer continue the relationship with him. After you have the talk, cease contact with him until you've both been able to move on.
Find a quiet place to have the discussion about breaking up. Make sure you won't be interrupted and that you can both give one another enough time to truly talk through things until you've reached some sort of resolution. Ask if you can meet him at his house so you are able to leave if you feel threatened or need to get out of the situation for any reason.
Explain that, because he cheated, you are no longer able to continue the relationship with him. Convey the reasons why it's hard to trust him any more after the infidelity. Explain how the cheating affected your ability to love and care for him in the way you once did.
Be firm in your decision. Your boyfriend may try to ask for another chance or justify his behavior in order to salvage the relationship. Listen to him, but don't allow his arguments to change your decision. Explain that, no matter what he says, it will not change the fact that he cheated on you.
Avoid insults, name-calling or getting angry, if possible. Stick to the facts and focus more on what was wrong with his behavior than your feelings about him, the person he cheated with or why he did it. Keep the discussion simple and avoid rehashing all the details of his betrayal.
Explain that you'd like some time and space to move on, and tell him it is best that he does not contact you for at least a couple of months. Ask him to respect your boundaries and give you the space you need.