My mother has had ear fullness for many years. She had seen 4 ENT's and spent $5000 trying to find out the cause of her symptoms with no diagnosis. One day she mentioned her ear fullness worsened drinking coffee. I thought that was very strange and looked up google and found her condition pretty quickly: Secondary Endolymphatic Hydrops (SEH). Here is some info on that condition from vestibule (dot) org...it is a very long article but here is an excerpt...
Endolymphatic hydrops may be either primary or secondary. Primary idiopathic endolymphatic hydrops (known as Ménière’s disease) occurs for no known reason. Secondary endolymphatic hydrops appears to occur in response to an event or underlying condition. For example, it can follow head trauma or ear surgery, and it can occur with other inner ear disorders, allergies, or systemic disorders (such as diabetes or autoimmune disorders).
Symptoms typical of hydrops include pressure or fullness in the ears (aural fullness), tinnitus (ringing or other noise in the ears), hearing loss, dizziness, and imbalance.
DIAGNOSIS AND TESTING
Diagnosis is often clinical—based on the physician’s observations and on the patient’s history, symptoms, and symptom pattern. The clinical diagnosis may be strengthened by the results of certain tests. For example, certain abnormalities in electrocochleography (which tests the response of the eighth cranial nerve to clicks or tones presented to the ear) or audiometry (which tests hearing function) may support a hydrops diagnosis. New research has shown that MRI with contrast in the inner ear can give a definitive diagnosis of endolymphatic hydrops, but likely would not be able to differentiate between primary (Meniere’s) and secondary. This is most commonly not used because clinical diagnosis is often accurate."
Have you had a tilt table test or been tested for nerve damage (They hole punch your leg a few times for a biopsy)? Since you say it gets a little better when you sit or lay down, that sounds like POTS or some other manner of dysautonomia. You should monitor your blood pressure and heart rate daily, sitting, upon standing, and after standing for a time before your appointment so that you have something to bring to the doctor. Ask the doctor your seeing specifically about aforementioned tests and dysautonomia before making an appointment because most neurologist will not test for this and do not treat this. There are only two doctors in my entire state that have the equipment to test for dysautonomia related conditions. I have an Omron that tests blood pressure and heart rate and syncs with my phone to make it easy. Regardless of whether or not it's dysautonomia, I find that the best way to figure out things like these when doctors aren't being helpful is to start a health journal. Write down when you're symptoms are at their worst during the day, what did you eat/drink, were you talking/walking/studying, what was your HR/BP at this time compared to when you're feeling your best. Taking a holistic approach to your health is also advisable as you attempt to determine what's wrong. All your smaller systems- renal, lymphatic/immune, endocrine, play a large role in how you feel overall and small things can tip the balance with them.