I too have felt the same way at times about my bf. He was with his ex gf for three years and they lived together and all that. She moved across the country. But i felt like well what happens if she decides to come back.... But then you just have to hope that you can trust what they tell you. I would think that since she was open with it from the beginning that it probably is what she says. If she didn't want you to know she wouldn't tell you about it.
Good Luck. I understand how hard it is to trust people i have the same issues!!