What can cause constant migraines, mental issues now seizures. high platelets 2 yrs. recent test platelets 565,000, monocytes 1.0, low mch. next step?

Discuss with doctor: The findings you describe are consistent with "essential thrombocytosis", a condition in which the bone marrow produces too many platelets. Cause is unknown; it may be genetic. The "next step" should be a discussion with the provider who will be taking that next step with you-- along of course with parent(s) or guardian(s).