What do you hope to gain from telling her that you
used to like her? How will it help you or her? If you have really given up on her, then why bother? Unless you really haven't given up on her...
Why did you give up on her?
If the topic comes up when you and her are talking, then fine, let it come out like normal conversation. Other than that, old feelings shouldn't bug you. Not to mention, if you start a conversation with her with the idea of "confessing your past love", she will think you are doing it to affect her feelings somehow. Which you probably are.