I'm stuck in an "I love you" predicament?

Y'see, this girl and I who've been really close friends for a long time just started going out. We've been saying "I love you" in a platonic way for a long time, and she still does--so now comes the time where I really think I am in love with her, but all those "I love you"s might not mean anything. What do I do to tell her?