I am a 26 year old, 285lb male who was recently in the hospital for attempted suicide. They did a CT scan and radiology determined that I have a Posterior Cranial Fossa Arachnoid Cyst, and I have been having headaches for 20 years straight. The doctor never told me. Now I am my balance continually, having nausea in the evening, and severe troubles focusing and able to remember. I was diagnosed ADHD and take Amphetamine*Salt 90mg daily. I also take Seroquel for other reasons at night. Is this cyst the cause of my problems?
Unlikely.  Arachnoid cysts are present when you are born as they are a result of a malformation of arachnoid tissue.  They can be massive but are always asymptomatic.  You should look to other areas as the source of your symptoms