I am in the same boat as you-I am a 20 year old female and I have been constantly dizzy for a few months. I have had tons of tests, I'll list them, you might want to look into them if you haven't had them already:
CT scan of sinuses
MRI of head
EEG of brain
Lyme Disease
Thyroid
Diabetes/blood glucose
white cell count
Epstein-Barr
basic bloodwork for nutrient/vitamin/mineral levels
All of these tests were negative/normal. I have seen my regular doctor, an ENT (he said nothing was wrong with my ears), a neurologist, and am now seeing a chiropractor. I saw him for the first time yesterday and he said it seemed that my neck has suffered some severe trauma (which is possible, I work at a farm and am always hitting my head/getting bounced around), and that the knots in my muscles are pushing on a certain group of nerves in the neck and could possibly be the cause of my dizziness (he said he honestly thinks its about a 50/50 chance that this is the root of my problem). So he is going to give me a series of adjustments and ultrasound therapy to see if it makes a difference. I have tried a bunch of different medications as well, none have made a difference. I am still under the care of my neurologist, she has me on nortriptylin to see if I have a migraine variant (meaning she has no idea). I hope you get some answers soon, it really stinks to have to put your life on hold. If you would like to talk, please email me, its interesting to find someone else with my same problem.