He wouldn't leave me for her?

although my boyfriend says he is bored in our relationship he didn't leave.Also i think he was having a fling, where he would disappear two fridays out of the month.He could never stay away from me for more than two nights.partly coz he loves our life!! if this woman existed she never called him so i don't know how they kept in contact.but one minute he was moody and withdrawn with me.then for a few days he was so down and worried about something. now he is attentive and loving and back to normal.He wants us to start going out together as a couple again.I suspect this woman left my man.why is this?