Remove your personal need for her reaction to end with a certain outcome. Tell her you love her, but don't pressure her to tell you she loves you, by asking her if she does, or even pausing after you say it and waiting for her response. And tell her why you love her, something about her personality that she values, and then just move on to the next topic so it seems like something you almost accidentally said. Moving to a more serious relationship isn't decided by a contract negotiation. It's something that happens through body language and emotional interactions.
tomcrist1949