Going through with Divorce

Hey everyone. My husband of four years had a make out session with an 18 year old at work. He is 25, I am 27. He's cheated on me before, with my friend, a year ago, along with other younger girls.
Why is it so hard for me to leave? I've confronted him, I told him I'm leaving. But I feel guilty, and like I'm doing something wrong, or giving up. I love him so dearly. We've worked very hard together the last year to regain trust from previous affairs-and this recent act showed me he will never change. Any success stories out there? Something comforting in this time? I'm so afraid of the future.