All the things that make you feel safe -- financial security, perhaps, or a committed relationship, or an orderly home -- will be tossed on their end in this relationship. This person will enter your life and begin to dig beneath the surface of all your carefully established structures, and you may feel simultaneously excited and scared -- as if they've come into your life on purpose, to challenge all the structures that you've built and now take for granted as necessary and right. In a sense, that's exactly why this person has come into your life: to challenge everything you think is essential to 'proper' living.
After all, these structures you've established aren't a reflection of external reality so much as they are a response to your internal climate, including your insecurities that have developed over lifetimes of hurtful and even traumatic experiences. If you aren't aware of these insecurities, however, you'll likely begin to look at your partner as a destructive force rather than just a challenging one. You think you need these external types of security, but they'll push you to develop internal security -- and therein lies the potential for both the greatness and the frustration of this relationship. If you can learn your karmic lessons in this affair, you'll be able to begin establishing a sense of strength within yourself that will never again be upset by any external condition or occurrence. But if you keep trying to exert greater and greater control as this person tries harder and harder to force you to face your own demons, you'll likely just drive each other crazy, and neither of you will learn anything of value in this regard.