"If you're having any doubts,
you should call off your wedding."
Here's why it's so wrong:
1. It's not helpful. It's condescending and also stating the obvious. (They've been down this road many times before in their own mind). And your saying it out loud is just not helpful. In fact, it just makes them feel worse for struggling with their feelings.
3. It's alienating. You're the one who's going to be alienated from his or her process of figuring it out if you make a bold suggestion like calling off the wedding. You've come across as judgmental and so have just made yourself an unsafe person to confide in.