The Help You Need
If the problem truly is that you're just super busy, then my best advice is to schedule a time for the two of you to have sex.
The truth is that life is hectic and it's really hard to make intimacy a priority, BUT you must. Many people, and possibly your spouse, only feel loved when they are touched or have sex. Neglecting this need is a bad idea.
But what if the problem is really that you're just kind of "over" sex and have no desire? Here are a few things to try....
To end the affair, I suggest that you and your spouse sit down and write a letter together to the other person (affair partner). The letter should mostly be written by the straying spouse (the one who had the affair) because it's critical that he/she starts to take responsibility for the choices that were made.
I also want the straying spouse to "own" this letter and truly feel what he/she is writing. The letter should be short and clear, but not emotional in any way towards the affair partner.
Here's a sample break-up letter that I recommend my clients send (via mail or email) to their affair partner...
Most of the time, if we keep fighting about the same thing it's because we never get the recognition or the acknowledgment we're craving from our spouse to let it go.
Or it's because we never get down deep enough and talk about what the fight is REALLY about. On rare occasion, we fight about something that just is not fixable, and instead of fighting, we have to find a way to talk about it that feels comfortable to both of you.