Are girls who had bad ex-boyfriends more likely to keep up their guard in new relationships?

Talk to her. She may just be holding back her feelings and is afraid of getting hurt. Don't hold it against her. Not all girls are the same. Be patient with her and realize, she isn't doing it on purpose - it's just that her life experiences have made her this way. She's probably a really great girl and worth the wait and the work. Besides, if you want a girl that is totally open with her feelings, then you're more than free to go be with one but, you might be missing out on a totally GREAT person! Take your time with her! GL!