Having been cheated on in my former marriage, then applying some "grown-up" logic to it, and looking at it from an outsider's point of view and analyzing the reasons he cheated, it wasn't for love; it was for sex and a plethora of his own phsycological issues (his family history crap).
There are a lot of reasons people 'cheat'; it's not always b/c they don't love their spouse. I'm not saying it's right, or that I condone it. But once I understood that, it made it a lot easier to not remain pissed off at him for it and move on in my own life vs. dwell on the past. Not understanding the reasons is why so many people remain pissed off at their exes and calling them a marital dirtbag ... which they are, but it's what causes them to hold on to the grudges created as a result. Once you can let go of the grudge and understand the "why's", I think you can re-open your heart to that person and together, work things out -- if you're both committed to the notion. It won't be easy, and a counselor may be necessary, but I don't think it's impossible.