It's OK we're all humans... Confused and confusing by nature, I guess. And I firmly believe that your problem is not her because you do seem to love her (if you didn't you wouldn't have the days when you love her). Your problem is you. Have you ever thought that somewhere in the beginning of your relationship you overdid it and put her on the pedestal? If that is so, then those feelings are getting back to you in a form of a disappointment. You may end up breaking up with her, but I doubt you will not run into the same exact situation with your next gf. The moral of the story is that you need to learn how to be content and learn to live with somebody's imperfections (i.e., her plainness at the moment). And the fact that you do miss her when she's not around you says a lot about your feelings. If you didn't love her, you would hardly wait to break up with her and not going on with the same old story.
Like with everything in love, listen to your heart and yes a few days break like >>6 proposed might be a good idea. This situation comes out usually from depression and reasons are not always so obvious (so it might not be her and her behavior that's causing all the hassle you're going through). From my own experience, I can tell that whenever I felt under pressure from work, school I would totally forget about the loved one.
Take a few days break, look into your life and find the reason.
Again, I hope this turns out great for both of you. Good luck.