It's difficult to fully be yourself in this relationship, because there is so much deep-rooted conflict between you. Even when you first met, your partner, especially, felt the effects of old, unconscious wounds rearing their heads in the present. You may both feel as if you've come to this relationship for a reason -- that there must be some meaning in why you're going through such pain together.
You have some painful deep-past issues that come to the foreground through your interaction with this person, and transformation is needed. This relationship is an opportunity to work out old karma. They find that your evolution as an individual is in direct conflict with their individuality, or their idea of who they are. Resentments build up as criticisms lead to tiffs, tiffs erupt into arguments and then intensify into full-on, angry fights. You're two different people; that's unavoidable. But this particular relationship doesn't serve either of you as individuals for long. You find that over time, you begin to grate on one another's nerves; the smallest disappointments seem huge, the most minor infractions suddenly enormous. You, especially, make compulsive demands on your lover regarding their behavior, even their feelings -- which can't and shouldn't be controlled by another person. This is a very difficult aspect to work with, and requires much patience and compromise on both your parts.