I don't know that I would call it the "worst", but rather the "hardest", and that would be the meshing of two people into one life.
When you share your life with someone so intimately, the intertwining of the two of you sometimes results in... knots.
The fact is that even though you are sharing your lives with one another on the deepest level, you are still two totally different people. You have different personalities, different ideas, were raised differently, different wants, different don't-wants, different likes, different dislikes.
Getting all of those things to successfully merge is hard, H-A-R-D, hard.
It's a matter of compromise and of give-and-take. Sometimes you give more, sometimes he gives more, sometimes you're both trying to take and no one wants to give.
Sometimes... you remember how nice it was back in your tiny one-bedroom apartment just you and your cats when you didn't feel obligated to ask someone else's opinion before you made a life decision.
But mostly, you figure it out. You do the dance. You give, he takes. He gives, you take. You both give and the kid takes.
And sometimes it sucks, but mostly, once all the dust has settled... those knots? the ones that got in the way when you were trying to gracefully intertwine your lives? Yeah, it's those knots that made the two of you stronger together than you were apart.