It is probably not about you, so don't beat yourself up about it.
Chances are, she may have been extremely hurt in the past and is scared to open herself up to the vulnerability of getting hurt again. I know I am like this. I have not been able to let anyone in since I left and divorced my ex a few years ago.
Give her space and time. But, maybe from time to time do something special for her to remind her that you are there and want to be in her life. Don't go overboard, tho so she doesn't clam up further. Hang in there. I hope things work out.