There is a shit load of guys out there that are desperate to the point, where they'll take any girl as their GF. A big challenge as an average guy is to convince the girl that you're not one of those guys.
While she enjoyed the "forever after" fantasy in the moment, I'm guessing that afterwards she thought that you're another one of those guys. Falling immediately for a random girl you just met.
Even tho women will tell you that they want someone that is serious and wants a relationship .. The point is that you're supposed to say "I definately want a relationship", but add a "with you" to that sentence and you'll likely get a relationship.
I suspect that this might have killed you off:
"Oh, ok I guess we're doing this."