One person can't fill a million different roles, not perfectly.
You see so often in relationships, it starts as lust and infatuation - you're drawn by the mystery of the unknown, the process of discovering someone new, the pleasure of a new experience.
Then it dulls and gets diluted by the administration and interdependance that builds over time.
Stay separate. You don't need anyone, no one else is your life.
Have your own time. You have better things to be doing than spending all your time with one person.
Have quality time. When you have moments, know what you want and make every movement, word and thought about that.
I think that last point is important. Especially when you're living with someone, it can be so hard to determine what time is quality and what's not. You lose track. You're there all the time. When you're reading, when you're studying, when you're making your food - they're always there. Are you spending time together? Not really.
Here's an experiment I'm going to try - don't be in the same room with someone if it's not time you're deciding to spend with them.
Let's see if things improve ;)