E) break up with both of them. If Lance wants to cheat he can find someone else. Tell Susie. Hope that she's willing to forgive you. Spend some time forgiving yourself. Optionally, explore relationship anarchy and/or open relationships instead.
Cheating on someone is devastating for them when they find out. Unless you are a cruel person, it will hurt you too when your loved one finds out about your betrayal. But not as much as it hurts them. Stop it. Susie (and Lance's girlfriend) both deserve better.