&gt; I feel as though if I was going to marry her I would have already asked.

This sounds like you are grasping at reasons you yourself don't understand.  I'd leave this argument out of the discussion. 

Before you give her 30 days notice, check your state's laws. It may be longer.  Also, she just move in there.  Does she have the money to find another place right now?  I will bust your chops on you about your timing, though.  You must have invited her to move in to your place, so she has a pretty good reason to believe that the relationship is going towards marriage and children, and you've even stated that it's in your future plans.  Her POV could be that you're just dragging your feet when its what you both want.

Could the co-habitation be getting to you?  A lot of guys have issues when the SO is suddenly there all the time, and there's no other apartment where she'll go home to. It inevitably leads to some friction, and seeing other women as more appealing is common during this time of tension.  

Good luck, she might not be the one for you, but don't waste what you have on feeling restless.