At some point, you will mirror each other. They call it the Siamese twin stage. Unconsciously, you become your partner. You begin to like what he likes, eat what he eats, talk, walk and even dress like him. You make decisions based on "us" not "me". And that actually feels good. And right.
You can keep on doing it for as long as you can help it.
Until one day, perhaps due to some hormonal cycle, you look at your face in the mirror and you don't see the you that you knew before you knew him. And you wonder if the new you is better or worse that the old you. And the answer to that question will either make of break you. It's time to deal with the Rude Awakening.