Most Helpful Guy
Long term? If she's been with me long term, she has no excuse to cheat. I'd break up with her. If you cheat and he finds out, that's not a mistake. The relationship was the mistake. You wouldn't be concerned about his feelings, you'd be concerned about losing his attention and losing his love. Because if you're willing to cheat on someone, you didn't love them that much anyway, just the attention and whatever you were getting from them.
Cheating isn't acceptable even in short term. If you are in an open relationship, that's different and the only exception. But there's no such thing as a truly "open" relationship, is there? Someone's always going to get hurt or jealous about the other. So if you cheat, you're screwed and would deserve it. The worse thing that happens to you is you lose someone who loved you, and you gain someone who is willing to help you cheat and cheat on you. No matter what, you lose if you cheat.