This comes to me from a friendly reddit user sjogerst so big ups to him! Also if you want to watch the whole theory in video form check it out BELOW
Ok, now that all of that is out of the way let's dive into this theory!
Our hero Luke Skywalker is headed down the Deathstar's trench at full throttle. The rebellion is counting on him. The Galaxy is counting on him. He has the only functional X-Wing left in the fight and everything rides on him nailing a 2 meter exhaust port with a pair of Proton Torpedoes he's never fired, except maybe in a crash course simulator. Its going to be hard to hit, nearly impossible. The squadron leader, perhaps the most well trained pilot the Rebellion had, already missed his mark and is now a carbon stain on the side of a Turbo Laser turret.
So first let's examine Luke's current state of mind. A day or so earlier he stumbles across the charred skeletons of the only parents he ever knew. The fate of the entire galaxy now hinges on him at this moment in the trench and is he has spent the last half an hour watching his childhood heroes die one by one at the hands of the Imperials. Not only are most of his heroes dead, but one of his childhood friends just bought the farm just to buy him enough time to finish his attack run. Now they are coming after him. Not only are they coming after him, but they have just hit his trusty astromech, R2. Luke's already diminished pool of friends is rapidly dwindling. He is surrounded by Imperial troopers that keep killing everyone he cares about.
It was about this time that Luke was making his final approach on his attack run. This is when Obi-Wan pipes up and is like "Hey man, use that Force thing i mentioned earlier." To which Luke is like "okie dokie". And at which the Rebellion is like "WTF traitor! Turn your computer back on!"
Han and Chewy show up and take a bit of the pressure off but here is the big moment. He MUST hit that target. To do so he has to use the Force, and not just a parlor trick to mess with a storm trooper's mind.
Real Jedi spend the first couple of decades of their life training to use the Force. It takes discipline, will power, and control. We watch Luke struggle in Empire Strikes Back to move some rocks and nudge his X-Wing but why do we see luke struggling to learn the ways the Force in The Empire Strikes Back when it seemed to come naturally to him in A New Hope?
The answer is simple. Luke was using the Darkside when he blew up the Deathstar. I dont fault him of course. I mean, he didnt even know the Force had two sides, let alone that he should try to have a certain emotional detachment when using it. Of any character is this movie, with the exception of Leia losing her planet, Luke has to have to some serious RAGE brewing under that calm demeanor at that moment. Yoda talks about the darkside being fueled by fear, aggression, anger, ect. I contend that the emotions that define a Darkside user perfectly fits Luke's situation:
A powerful natural force user, under great stress, slowly watching his hero's and childhood friend get shredded by faceless imperials, while a Galaxy counts on you to make the perfect the shot, with weapons hes probably never used, flying a ship hes never flown.
Fear of failing, Anger at watching everyone he cares about die, all consuming Rage. You get the idea.