No matter how long you've been going out with someone or how well you think you know them, they will never fully understand you and you will never fully understand them 100% of the time. You can't read each other's minds, and there will be lots of misunderstandings, periods of confusion, frustration, and hurt. This is a part of life. You are two separate people and that will never change.

If you learn to accept this, it becomes easier to be in a relationship. I think a lot of the frustration people feel is due to the mismatch between expectations and reality. Also, know you are not easy to live with. No matter how easy going you think you are, you probably have quirks and habits that drive others around you up the wall.

Worst advice: expecting that a life event of some sort will change the other person. Like getting married or having a baby will suddenly make the other person into what you want them to be. People change if and only if they truly want to. And even then, it's not easy.