Okay I am ready, I am posting my answer here;
A pineal cyst generally does not cause headaches, but rarely it can cause headaches and other symptoms too
Pineal cysts most often do not grow in size, and remain at the same size so they are most often not dangerous
Pineal cysts are not considered fatal
Rarely pineal cysts may grow and if that happens then it may require some kind of surgical intervention to remove the cyst
Regarding small vessel ischemic disease can produce the various symptoms you are having including memory loss, loss of words etc. The treatment for ischemic small vessel disease of the brain includes control of blood pressure, blood sugar, cholesterol etc (you seem to have high cholesterol & prediabetes, so these need to be controlled), and stopping smoking, and daily exercises, eating healthy diet like Mediterranean diet etc.
You may also try medications like Aricept or Namenda etc which can help with improving your memory and other cognitive dysfunctions you are having
And regarding the headache you have mentioned; this headache is unrelated to your pineal cyst. This has components of some tension headache along with some migraine features too. You may try stronger headache meds like Lortab or Vicodin. Meds like Amitriptyline or Gabapentin too can help for this type of headache
And coming back to the pineal cyst, these cysts are not considered as fatal, and I can strongly reassure you in this regard. 3 x 5 mm cyst is definitely small and unlikely causing your symptoms, and this sized cyst should never be fatal too. You need follow up MRI of the brain once in 6 months to a year or so, to make sure the cyst is not growing, if cyst grows then it has to be surgically treated. However the good news is most of the pineal cysts remain of the same size and are never fatal
Please let me know if you have any further questions or if you want further clarification and we can discuss this further.