First and foremost, you need to be direct but sympathetic. If you are the person have decided to let it go, your partner may not think the same way. You need to step into his/her shoes and show some compassion. Of course, we are discussing people who deserve the compassion, not someone who are mean and really should be kicked off quickly.
Second, not all breakups have to be put up in a harsh way. You can sit down with the person and start by talking about your good time. Anyway, no relationship is totally black or white. By starting with the good past, you help ease the blow a little bit.
Then, you need to explain why you believe you are no longer suitable to be together. Your explanation needs to be clear and to the points with reasons and facts that are hard to be beat. Don't be vague too general in your explanation. That way, your "after affair" relationship will be even harder.
Finally, and maybe the most important thing, take responsibility for your part of the breakup. It’s never just one person’s fault. Being honest about what you did wrong will make the sad reality a little more acceptable.
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