Relationship is like education. In education, an individual needs to acquire the knowledge of a particular or several disciplines to aid the mastery of what was taught. This acquisition process is usually in stages and it could span decades. That's why is unusual to be awarded a Ph.d on the first day of elementary school. I believe people need to have such acquisition process in relationships. The knowledge to understand relationships is not in the classroom but rather in experiences.
A lot of people are in messed up relationships. The become messed up themselves after the relationship is...over. At that moment, some wonder what went wrong, who was wrong, and ask "I can't believe I could be that stupid/gullible?"
Relationships are complicated [at times] but it is not rocket science. I observed people that suffer withdrawal symptoms aftermath of a wrecked relationship lacked 3 main experiences. I know that the acquisition of these experiences will boost the self esteem and confidence of anyone in and out of the relationship.
Experience 1: To have a strong affection towards someone that doesn't feel the same way.
This experience is acquired early in life and mostly by guys. Such guys developed such attraction towards someone and got turned down in the most humiliating way ever. I don't include crush on celebrities, but people we are accessible to.
The knowledge acquired is that not everyone is for everybody. The individual realise that the love for another does not translate to the feeling being reciprocated. You either become friendzoned or avoided.
People who acquire this experience in their twenties, thirties, or forties react to such rejection with a negative approach. They tend to believe "something" must be wrong with them INSTEAD of "it wasn't meant to be".