Try fitting in with yourself, rather than with a random group of others.
Once you're comfortable with yourself (could take years) try fitting in people you're comfortable with around you..
I had (still lingers a little if I don't catch it) this problem mainly with relationships.. I always focused on being who THEY wanted me to be... this is destined to failure because you (1) don't take care of your own needs & (2) you're being fake. The first is actually the worst because even if you did fit in entirely on someone else's terms you'd run out of your own psychic energy and run yourself down. Slowly slowly I learnt what my needs are, what things strengthen me in my life. To do this you'll have to get to know yourself better. And this could be scary and even disappointing, specially for a "good" Christian... However in my mind its worth it..
Here a wrote a song about this endless cycle of trying to fit in with someone else, the inevitable friction and then trying again...
http://bit.ly/2nPrBEm