The question you have to ask yourself is whether this incident is totally unlike her, or one instance of lies and disrespect among many. Relationships can get past affairs if iit is an isolated incident.

That said, I think it is important to look at the context. You aren't married yet and it will be easier to end things now than after you are married. Also, it is relatively early in your relationship and she can't handle it. It isn't like you have been married 10-20-30 years. Also, I'm assuming, you have no children with her yet, making it easier to get out.

If you don't have a therapist, now might be a good time to get one. You need support at this difficult time.

Best,
EJ