If you want to be happy for the rest of your life... It has nothing to do with pretty or ugly.
It is all about respect and admiration. You must also avoid:
Choosing the wrong partner. A lot of people choose somebody for the wrong reasons. Have a solid foundation in the relationship so that the partnership can continue when some of the other factors start to erode.
Lack of communication. It's important to resolve disagreements immediately and not allow resentment to build. Take time to right any wrongs and never go to bed angry with each other.
Do Not ignore Red flags
You have to be diligent in making sure that your relationship hasn't gone south. Aberrant or unusual behavior, like changing passwords, making or receiving calls at hours that don't seem appropriate, or a sudden wardrobe overhaul, as tell-tale signs of an affair.
Sometimes if you catch it early, you can resolve the relationship. The longer that extramarital relationship continues, the harder it is.