Your 22 year old boyfriend of a year and a half, has been looking for romance, sex and hookups outside of the relationship. You caught him. He’s admitted it. He says the reason for his behavior includes the fact that for him the relationship went stale. You asked me how it could’ve gone stale for him without your realizing it. The answer is that just because one person is happy or content in a relationship doesn’t mean the other person is. That happens all the time. As for his other reasons, I think they’re just excuses. The bottom line is he’s not interested a relationship that is honest and open, and if you hadn’t caught him, this probably would have gone on indefinitely. I appreciate that he wants you to stay with him, and that choice is yours. But if you do stay, you have to understand that he may simply be too young to want the type of commitment that precludes him from playing the field. He was getting something out of his extracurricular activities that he didn’t get in his relationship with you, and this may have been a spark and excitement that fades over time in most relationships — and requires relationship work to nourish — but it could also have been him not wanting to settle down. You have to figure out which of those two it is. I’m sorry you’re hurt, and I hope this helps.