But you aren't supposed to say it. Just let her go on her way thinking everything is wonderful and all 'ment to be' and BS. Neither break her down nor build her up, both lead to trouble. Just become as non-commental and non-commiting as possible. The best answer for what happened (at least, as I see it) was to say the 'I love you too' and then change the topic, and change it to something she likes (shoes, clothes, celebrity gossip, or whatever the hell she's into). The point is, say as little as you can and then change the damn topic before she goes fishing for more!
At least, that's how I'd have handled it.