I'm sorry to hear that. Their loss, right? There's lots of reasons for it, i suppose. But in the end, it's for the best. You deserve that lifelong commitment and dedication. That special person is out there. Just keep your head up so you don't miss them.
Two things: you became boring. She had an affair. Don't get suckered back into it. She will do it again over time, in spite of her convincing promises she won't. But I bet you will fall for it. You will take her back after her begging and groveling and you will live to regret it. She will then break up with you for good. You will be a basket case until you rush into a rebound relationship that you will regret. The only right answer is to dump her now and don't look back.
Their probably not in love with this person in as much as they've gotten bored or just fell out of love. Sorry, I know it's a painful situation. They might come back. But then u need to ask urself if u take them back what are the chances of it happening again? Life is too short, it ran it's course, find something to keep u busy till u find love again. It will happen. Good luck....;)