Michael never intended to tell her the truth.
Was she blind? We've often discussed that point. I believe the consensus is that she wasn't so dense as to be unaware of what he was doing, but she had a blind faith in her ability to change him. At Anthony's party, when she and he are dancing, she pretty well summarizes her growing disbelief in him. Then, in the scene after the Tahoe shooting, when she's clutching Mary, note the look she gives Michael. If looks could kill...