reducing fear when cycling?

she isn't like mega unhealthy but I remember in the summer she did streatham to wood green via hackney with me(15miles?) and couldn't/wouldn't cycle back to streatham the next day(so I had to get a bus up and cycle the other bike back for her) part of that was the mtb is a pile of shit but when we had city bikes in antwerp/paris/ghent/places the distance was about the same.

Have you considered that maybe she's just not into cycling?