I don't think it's a matter of "re-falling" in love..
I think you either continue to love them and forgive them, or you try to shut that love down and move on. It's hard.
In our hearts, if we truly loved this person, we may understand what drove them to it - usually some insecurity on their part. If you can get to the source, the relationship has an opportunity to grow deeper than it was before.
But like reeltaulk said, that puts you in a vulnerable position, and depending on who you're dealing with, that could be taken as a sign of weakness - while they may appreciate it, they may walk all over you... if it's that sort of person who is not mature enough to develop a real relationship - it's best to move on. You can love them, but there's a limit to their ability to love.
Everyone has the ability to learn and grow, the best thing to do is talk about it. That's what these hard things we face in life are all about - opportunities to take life to the next level.