It's because the saying " once a cheater always a cheater" it is hard to believe once someone can easily break trust with you they won't do it again.
It would be a horrible relationship for her if she always doubts you.
Have you come totally clean on your whole cheating past with her to her? Has she asked to see your phone logs, texts, emails?
I don't think you can do anything to make her trust you, she just has to learn to re trust you on her own. Sorry.
It's not what you have, it's what you do with what you have that matters.