Before we even get into what you should do in your relationship, let's start with what not to do.
"Don't ever be with someone because someone else pressured you to. I got married the first time because I was raised Catholic and that's what you were supposed to do. Wrong. I got married the second time because I was miserable and lonely and thought having a loving wife would fix everything for me. Also wrong. Took me three tries to figure out what should have been obvious from the beginning, the only reason you should ever be with the person you're with is because you simply love being around them. It really is that simple."
– Greg
When I sent out my request to readers for advice, I asked people who were on their second or third (or fourth) marriages what they did wrong the first few times.
By far, the most common answer was "being with the person for the wrong reasons."
Some of these wrong reasons included:
Pressure from friends and family.
Feeling like a "loser" because they were single and settling for the first person that came along.
Being together for image — because the relationship looked good on paper (or in photos), not because the two people actually admired each other.
Being young and naive and hopelessly in love and thinking that love would solve everything.
Everything that makes a relationship "work" (and by work, I mean that it is happy and sustainable for both people involved) requires a genuine, deep-level admiration for each other. Without that mutual admiration, everything else will unravel.
The other "wrong" reason to enter into a relationship is, like Greg said, to "fix" yourself. This desire to use the love of someone else to soothe your own emotional problems inevitably leads to codependence, an unhealthy and damaging dynamic between two people where there exists a tacit agreement to use each other's love as a distraction from one's own self-loathing. We'll get more into codependence later, but for now, it's useful to point out that love, itself, is neutral. It is something that can be both healthy or unhealthy, helpful or harmful, depending on why and how you love someone else and are loved by someone else. By itself, love is never enough to sustain a relationship.