I believe those relationships fail because the couple believes they're "one". But in my opinion that's not true, they're two individuals with a common interests and a common objective in life.
They may crack due to your partner act or reacts to a situation that you were not expcting he/she would react from the image of him/her you have in your mind.
Parents that have life-long relationships have learned to accept differences in their partner, and not always fight to the last drop of who is right or wrong. Like the media tells in the movies.
Tolerance and being humble just to ease the fights when they get to harder, I believe that's the keyword. And respect.
And then forget about those things and do not live with a guilty conscience. This is not a trait in modern couples I believe.
"E ao imenso e possível oceano
Ensinam estas Quinas, que aqui vês,
Que o mar com fim será grego ou romano:
O mar sem fim é português."