Vertigo,lightheadedness, for 5 years? help?

i have had chronic vertigo for 5 years, i am 28, and it just came on suddenly when i was 23. i have seen every doc imaginable, including holistic meds and chiropractors. i have learned through all of this that it is highly important to stress wether i am dizzy or lightheaded. I am definantely lightheaded, like when you stand up too fast, yes my blood pressure was checked when standing up and down and that was fine. I have a feeling of being "car sick" with pressure behind my eyes, yes my vision and sinuses were checked and no, no tumor. my doc has had me on valium and meclazine for treatment, sometimes it works, sometimes not. Lately i have been wondering if something could be wrong with my eyes, not my vision...anybody help!? it's to the point of desperation. i have had my hormones checked also. fine there too. like i said, tried everything and every specialist.