Why can't she say I love you?

My girlfriend and I have been together for about a year and a half. 4 months ago I finally told her that I love her. I knew that I did before that time, but early in the relationship she had told me that she once dumped a guy after he told her he loved her because she knew he did not and she did not love him. That relationship was her longest outside of ours and it only lasted 3 months.

She was happy I told her I love her and even said she knew I was going to tell her. But the thing is she won't say I love you back. She just says she cares for me or that she wants me in her life. I'd think after a year and half she would know if she loves me and be able to say it. Especially since we live together.

So explain to me why a woman can't say I love you... I don't want her to say it just to make me happy. But I'm not sure how to take it because I thought she would have said it by now. I do know her parents split when she was very young so maybe she has love issues?