From personal experience, I can tell you - this is so true. When you've been abused for so long, you just don't know how to take it when someone actually treats you the way that you deserve to be treated. You're always looking for the other shoe to drop, the demeanor to change, the lies to be discovered, for everything to fall apart.
You get to where you are so afraid of being hurt, you just close yourself off and don't let anyone get too close, and you certainly don't let anyone "in". It really takes a special, very patient, true love to get through - to help you tear down the walls, one brick at a time, until they are able to get close to you and for you to let them "in".