Ok so thank you for indulging my plunge into the depths of relationship doubt.
Despite my occasional moments (or days) of panic, I am a hopeless romantic at heart and even if it's naive I will always believe that my FSU is one of the few and far between special kind of guys. The kind that doesn't think spending a lifetime with one girl is horrifying.
I think when you are in a relationship for a long time it is easy to get too comfortable, and that is not a good thing. Comfort is good, realizing you've stopped trying is not good. I guess if nothing else a little paranoia keeps you on your toes.