I think what your going through is normal. After being through all tha she must have matured a lot. Sure people change, if you really want to be with her, then work on it. Maybe spend more time with her, do stuff that the two of you enjoy. I was in a three year relationship, and after a year and a half or so we fell into a phase where love didn't feel like love anymore. Not because we didn't love each other, but because the love had matured, and it wasn't as intense, as in the "honeymoon period". I thin it becomes easy to think that the love has "fizzed out" but I think after being through all that with her, your love can't just vanish.
You guys just need to rediscover each other. Unless like some other people have said, your not too serious about this girl, it doesn't seem like it though...so just work on it, and stop questioning it, cuz if you keep feeling your falling outta love and start accepting it, it'll only tear you guys furthur apart.
And since you know you cannot see yourself,
so well as by reflection, I, your glass,
will modestly discover to yourself,
that of yourself which you yet know not of.