First of all, you’re certainly not the only person to go through this kind of feeling.
You don’t have to be celibate during your journey, but you also can’t act in a way that’s going to hurt someone else. So first things first, if you’re not being honest with this other person, then you need to start there. A lie of omission is still a lie.
Cheating is not part of the twin flame journey.
Just because you are physically separated, it doesn’t remove or diminish your bond. Given the depth of this connection (and I mean the actual soul-level stuff, not just attachment), completely forgetting each other is basically impossible.
But that still is no excuse for causing harm to someone else. Too many people do this and rationalize it away. If you are causing harm to someone, you are causing harm. That is not the path to union. Nobody wants to think of themselves as the ‘bad guy,’ but you might literally be this person’s karmic to help them reach union if you’re physically or emotionally cheating on them.
If you feel guilty, it’s because you know you are doing the wrong thing and causing harm to someone who doesn’t deserve it. Either learn to truly surrender and move on from thinking of your twin or cut this person free before someone gets hurt.
You’re not a bad person for feeling this way. Twin flames struggle with a connection that nobody else could ever understand; however, if you keep this person around, knowing they’re more invested than you are, then you’re not acting morally.