Help with my marriage!

I don't know how to get my husband to take our marriage seriously! I have told him I'm miserable and can not do this much longer. He swears he will change but then goes back to his old ways. He has no idea how much he is killing me!! I have threatened divorce and he still doesn't change. Quick background: I just found out he cheated on me 5 years ago before we were married. He swears it was a mistake and a one time thing but he has not shown any remorse. He thinks I'm just going to keep putting up with his shit. I seriously need help because I can not live like this!! I fear that my only option is to divorce him even though I still love him. I definitely don't feel as strong towards him as I used to but it's so hard to go through divorce. I'm fairly young and we haven't been married long.