Knowing others is simple enough. Some people are better at reading others, but generally it's not too hard to understand most people. Knowing yourself is harder. Most people are too proud or too blind to see the problems with themselves, hence they always make the same mistakes, act like a friggin idiot or a total asshole, and they're not even aware of it.
But walking the walk. Oh man that's so hard. Humans are strange animals, we're often compelled to do things we know we shouldn't do, and lack the will to avoid things we know will hurt ourselves and those around us. Take the most extreme examples. I know a guy that's had a heart attack which almost took his life, the doctor told him to lose weight and quit smoking. Yet the guy still smokes and eats crap everyday. It's amazing that as long as a gun isn't stuck to someone's head and death isn't staring them in the eyes, some people will lack the will for self preservation and go out and live a lifestyle that will eventually kill them.
Even I live a terribly unhealthy lifestyle. I smoke, I drink more than almost anyone I know, I eat crap and a lot of it everyday, and that's why I'm overweight even though I exercise regularly. Recently I didn't drink, ate right and exercised hard and lost 7lbs in 9 days. Which means living my normal unhealthy lifestyle I'm probably at least 30lbs heavier than if I changed it to a healthier equilibrium. Sigh, but it all seems so hard. It there was a God, he's got a cruel sense of humor. Most of the fun things in life are unhealthy, and the good things in life are generally painful or require willpower to stick to. Now most people would say, hey just find a balance and don't let the fun things in life control you. But if it were that easy then people wouldn't have problems with addiction, people wouldn't have problems losing weight, or problems with their marriage, or problems finding the right person.
Most of these issues come down to lack of willpower and living a lifestyle that goes counter to what we know ultimately is the best for us. Now with regard to relationships I can't give advice on how best to gain control over yourself, since willpower is something totally different from advice. All I can point to are the most glaringly obvious situations. People moan about not being able to find the right person, but even when they've found someone that's really nice they'll go and live a lifestyle which adds temptations and distractions to the relationship itself. If you want to live a fun lifestyle, then don't moan about being lonely.
You would think that going out partying and meeting more people means more opportunity to find the right person right? It's the opposite. The main reason people can't settle down in our society is not because they can't find the right person but because they are having too much fun. Damn those tough choices. Chocolate brownies or 6 pack, partying every weekend or settling down for good. Some people have the luxury of enjoying both worlds, most need to pick one or the other.
As for cheating. Well more than 50% of people have done it at some point in their lives, and then justified it in one way or another. We all know it's wrong, but most people still do it. Hey and just remember even powerful people lack self control, just look at Bill Clinton and Eliot Spitzer. They say all's fair in love and war, and you can't look down on people for cheating when so many have done it. But if you're the one doing it, don't moan when going to bed alone.