This is the final installment and it comes from a guy.
Q: What is the biggest and hardest lesson you've learned in marriage?
A: Biggest revelation: it's not about me (i.e. It's about oneness - the two shall become one. It's about me loving and caring for my wife as I love and care for myself, learning and anticipating her needs, helping her to become all she can be - God's plan for her). In this effort I have to be ever ready and willing to forgive her (and seek her forgiveness), I have to swallow my pride, I have to put her first - sacrifice for her... All this was news for me, and is an ongoing challenge, but awareness and understanding is half the battle.
Hardest lesson? Putting most of the above to work - despite my wife's stubbornness/pride. But I'm figuring it out better now (with her help). The other is raising kids... It's a unique and awesome experience (that I would do again and again in a heart beat, but it's no walk in the park either).