Cheating on Girlfriend or Boyfriend= BAD!

Cheating sucks. If you're at a point in your relationship where cheating is too great a temptation to resist, just break up with the other person. Don't put them through the anguish of your confession, just break it off. Being dumped sucks too, but at least there's closure in that. Trying to stay in a relationship where the other person has cheated is absolute hell. All you can think about when you're around them is the mental image of them screwing this other person. It's just a prolonged state of misery, and nobody should have to endure that.

By the same token, if you've been cheated on, just dump the other person and walk away. The relationship is permanently and irrevocably damaged at that point, and unless you're married, a normal relationship isn't honestly worth the outrageous amount of time and effort it's going to take just to coerce the wounds into starting the healing process.

To summarize: if you can't resist the urge to cheat on your significant other, break up with them. They'll be hurt, but not nearly as much. If you've been cheated on, dump the other person and get on with your life. You don't need to stew over it for months and months, because unless you go into extensive therapy, the pain won't go away any other way.