I was hit by a car when i was little. recently discovered an arachnoid cyst within the posterior fossa, on the left side midline (3.1x1.9cm). could this be a cause of my extreme fatigue and brain fog?

Unlikely: These cysts are generally not symptomatic as they are often present for decades. Occasionally they can lead to bleeding around the brain, which often leads to severe acue symptoms (severe headache or stoke like symptoms).

Unlikely: Arachnoid cysts are not known to produce brain fog or fatigue. They are not known to be related to childhood trauma either. Recommend full checkup by your family doctor.