It's not you. Trust me when I say that you're not the reason she has walls made of concrete surrounding her heart and you're not the reason she keeps those walls up. But, you're the reason the barrier is starting to form cracks on its surface.
This scares her beyond belief. She's so used to being protected by the walls that have been there since before she can even remember, and now that you're taking them down, she doesn't know what to do. She's terrified of what you're doing to her, even though you're helping her. She knows this, but she will still do everything to keep her walls standing as tall as possible, despite your best efforts.
There's really nothing you can do until she deems you worthy enough to scale the concrete or to remove the barrier altogether. Until then, it will be tough, but you can still love her. There's just going to be some things about her and your relationship with her that are a tad different. That doesn't mean it's any less worth it.
She will try to shut you out.
She can feel herself getting attached, and although she loves it, it terrifies her. The last time she got attached to someone they took a chunk of her heart with them and left her to pick up the pieces they shattered in their wake of destruction. It's not personal if she doesn't want to always hold your hand or touch you all the time; she's just afraid to form another connection.
She will want to be alone sometimes.
The solitude of her barricaded heart isn't just a mental or metaphorical thing. There will be times where she won't text or call or want to talk to you. She does this to everyone though. Sometimes, she just needs time to focus on herself and her own happiness and there's nothing you can do but let her. If you try to worm your way into her isolation days, she will begin to resent you even more, not necessarily on purpose.
She will take things slow.
It may be a while before you two hold hands or kiss or do anything that involves her stepping out of her comfort zone. This isn't because of you and you should never think it is. It's because she knows that relationships aren't always about touch and intimacy of the body but rather intimacy of the mind. She will want to be around you and the rest will happen when it's meant to happen.
She will not be good at expressing herself.
She's not used to having people ask her things because she always thought nobody cared. Now that someone actually does care, she doesn't know how to handle it. She'll leave the figuring out to you and expect you to figure out how she's feeling. Talk to her about it and ask her to share what she's feeling because let's face it: you're not a psychic. Communication is key in any relationship and if you want the two of you to last, she will have to learn to talk to you about her feelings. Help her through it.
She will not start anything until you do.
This can get annoying, but her mind is in the exact opposite place. She will think it's annoying when she's the one who texts first, or makes any sort of move toward you. She will think that if you don't spark things, then you don't want to talk to her, and she'll think that if she sparks things, you'll only roll your eyes and try your best to ignore her. It's happened one too many times for her that now she has to think about anytime she wants to talk to someone.
But, she will love you more than anything.
It takes a special person to be able to knock down concrete walls, especially ones that were only there metaphorically. If you get that far, and I hope that you do, you'll notice the beautiful girl you spent all that time trying to find, and she will recognize how much you really do care and want to open herself up more. She will see that you're genuine and true and she'll want to stay with you for as long as she can.
I know the path to loving a closed off girl is difficult, but what journey isn't? You have to experience the rain before you get the rainbow, but if you don't try, you'll be stuck under a dark cloud forever. So, love the girl who was hurt one too many times, because then you can help her to glue her broken pieces back together and recreate the masterpiece she once was. It will all be worth it in the end.