I think there may be 2 different questions here. The first is about what cards might symbolize insecurity. I am not sure I'd agree that the Devil would have anything to do with insecurity. I would choose ill-dignified Wands or maybe an ill-dignified Emporer, even. I might also include the 9 of Swords or 10 of Swords in that category.
As for the reading you did, I agree with the poster who said that he might not like that she is either controlling or overly needy. Insecurity can push people to become tyrants in a relationship, to constantly need feedback, to need constant contact with someone, or to be jealous. To me, the Devil can also mean sexual stuff...and unlike Nisaba, I will say that maybe the problem is a lack of desire on either (or both) of their parts. Maybe she has body hangups and can't really "let go" when it comes to sex?