"Once a cheater, always a cheater." Sadly, we think that there's a fair bit of truth to the old saying. A handful of people will only cheat once and forever realize the errors of their ways. Certainly. But it does take a special kind of person to think of cheating on someone instead of just breaking up with them, and those people aren't really going to change.
She may not have cheated on you before, but if she's cheated on any of her partners in the past, then she'll remember the thrill, the ease of getting away with it, and the ability to quickly jump from relationship to relationship. It's just one of those individual traits that's hard to shake. She may not be doing it now, and she may stay true for years to come. But somewhere down the line, there's a high chance she'll slip up again.