If you have not told her how you feel, then perhaps telling her via text is not the best way. If you really have to text her, just talk to her about general issues, although you could ask subtle questions about her private life.
If a guy liked me, i would want him to tell my face to face, so that we could actually talk about it. If the relationship isn't going to work, we can talk without the awkwardness of meeting face to face again after a texting conversation. you could also call her... anything but text/ instant messaging.