Her ex probably lied to her so much about what he was doing that it's hard for her to believe you at face value now.
I think it might be helpful to give her some empathy, and then stick to your boundaries. If you stay strong, she will feel safe and be able to trust you.
WL,
Thanks for stopping by.... I think you are spot on there...
How are things in your world?
Take Care,
RMG
"The bad things in life open your eyes to the good things you weren't paying attention to before." from "Good Will Hunting"