Unfaithful
If your love life is boring or you have a problem with your relationship, you will not be able to fix it by adding another person to it. This will only complicate things and make the situation worse. The fact that you broke up anyway a couple of months later just proves the best option would have been to break things off before cheating on your significant other.
You might assume that your breakup was unrelated but I am almost positive that it has something to do with it. People think that they have been able to hide their indiscretions when in reality they are very obvious about what they are doing. Odds are that your partner knows about it.