No, it's not just "normal" for girls. Guess what, there's guys out there who rant and rave and we do the emotinal turning off.
You sound like a really sweet guy, but she has a little more growing up to do. Since you're both so young, I wouldn't be thinking about tying the knot at this point. A good marriage requires a lot more conviction that just someone pressing you into doing it. It's not cruel of you to decide that at this point this is not the best venue for the two of you. You'd probably be sparing each other a lot of pain. Only you truly know the answer to this though, so listen to your inner gut feeling, it won't steer you wrong. :-)