Most Helpful Girl
You can leave a relationship any time you want. If it doesn't feel right to you then you shouldn't stay with her. I would like to say though that no relationship is perfect or exciting 100% of the time. Every relationship is going to have low points or go through dull moments. Eventually you lose the "honeymoon" feeling when you're with the same person long-term. That's normal. If you truly don't feel like she is the right person for you though then you should definitely end it. It wouldn't be fair to stay with her when you know you don't feel the same.
Most Helpful Guy
If it don't vibe it's time to leave. You don't want to be stringing that girl along, get out and go find yourself some strange.