To me, it kind of sounds like she might have some sort of self-esteem issue going on. In my head, it would explain why she cheated, why she moved fairly quickly with you, and why she's now feeling like she doesn't love you enough. The reason I think this is because when I start feeling really bad about myself, I look at my husband and just question what it is that I do for him that makes him want to stay.

I think therapy is a good idea, as well as keeping the lines of communication open (you're doing a great job with this already). She's going to need some time to figure this out, but I hope she spends some of that time working on feeling better about herself. It's that corny cliche, but it's true. It's hard to feel loved by someone and love them back when you don't love yourself.