What the heck is existentialism?

This is just plain crazy, but it sure is fun.
Existentialism as defined in the British Dictionary is -- a modern philosophical movement stressing the importance of personal experience and responsibility and the demands that they make on the individual, who is seen as a free agent in a deterministic and seemingly meaningless universe.
Or you could go with Dictionary.com -- A philosophical attitude associated especially with Heidegger, Jaspers, Marcel, and Sartre, and opposed to rationalism and empiricism, that stresses the individual's unique position as self-determining agent responsible for the authenticity of his or her choices.
What this means is that you and I are distinct individuals trying to cope with a meaningless and absurd world, and the choices we make under these absurd conditions often leads to a crisis. In other words there is no meaning in the world other than what we give it even if that meaning is absurd.
This sounds to me like one should just forget what science knows about the universe and just think whatever one wants even if it doesn't conform to any known discipline. Maybe this is the definition of a free spirit. I realize that a lot of life is absurd at times, but many things can be explained or rationalized. Let's face it, philosophy, especially in the so-called modern era is often obtuse and irrational.
The main premise of existentialism is that existence precedes essence. What the heck does that mean? It means that we are individuals that act independently and according to our own ideas instead of being labeled or stereotyped. Or one could say that the world is absurd and has no real meaning and this absurdity could impact us at any time and cause us to be in a state of confusion and despair, what existentialists call an Existential Crisis. This sounds like a goofy branch of psychology.
Existentialists believe that people are not rational and therefore throw out positivism and rationalism. They assert that people make decisions based on subjective feeling rather than reason. In other words, they don't believe in truths because truth is rational and therefore absurd. Forget the Bible. Existentialists would consider the Bible a bunch of disconnected events without meaning or value.
Existentialists believe in human free will and that society is unnatural and that traditional religious and secular rules are arbitrary. They believe that worldly desire is futile. In other words, there is a sense of despair. One must acquiesce to the idea of existing in an irrational world and adjust accordingly.
I understand how this idea became more popular after the absurdity of World War I and II and the depression. If anything could drive people to think that the universe was irrational, those two wars would. Life took on an absurdity born of death and destruction on an industrial scale. This could explain why so much pessimism blossomed at this time.
A lot of existentialism crept into psychology and a branch that became psychoanalysis. Freud established this concept as a major treatment for mental illnesses. Anxiety, depression and suicidal thoughts were thought to be the inevitable result of bad personal decisions. In other words, the person's very existence with its associated freedom influenced their decisions. It was the job of the therapist to show the subject the consequences of this freedom to choose. Of course, we now know that many mental illnesses are the result of flawed brain chemistry for which many drugs have been developed to alleviate the symptoms.
The absurdity of life was certainly exploited by Monty Python's Meaning of Life and in the TV series Monty Python's Flying Circus. Watching them gave me an existentialistic take on reality and afforded me some very hearty laughs.

Thanks for reading.