Though it might not have been apparent until well into the relationship, you've likely realized by now that there are fundamental differences in your intellectual ideas that might create a real challenge to your connection. The ways in which you think about things have turned out to be at odds with where your partner needs to go in life, and staying together means a big amount of compromise -- which may be holding your partner back from meeting their goals. This could occur on a basic needs level -- such as, one partner believes strongly in family and home, while the other wants to pursue a career that will take them to the far corners of the globe, alone.
Or it could occur at a more philosophical or intellectual level, with your partner pursuing a life path to which you're ethically opposed. You started this relationship feeling as though it was brimming with possibility, but as time has gone on and you've communicated more and more about yourselves, your beliefs and your dreams, you've begun to feel less and less like you can relate to each other. How painful this can be, and how difficult, is determined by other aspects between you and by the depth of your bond when the realization occurs.