I understand what you're going through. Like you, I had such debilitating allergies and nothing seemed to help relieve the congestion. I was so desperate I tried some crazy things, like sinus rinses with baby shampoo. I was on Zyrtec, Flonase, Sudafed and Mucinex and was still miserable.
Have you tried Singulair? It turns out it really helps specifically with sinus inflammation caused by allergies. I added that to my regimen and within a day, I felt better. The key is to keep taking it even when you feel well. It prevents the inflammatory process from breaking down the epithelium in your sinuses, which snowballs into worsened allergies.
Also, most people don't know that a thyroid issue can cause sinus congestion. Most doctors don't even think about it since it's not a main symptom for most people - unless you already have allergies and then any added congestion means this becomes the MAIN complaint. A thyroid problem can also make your allergies themselves worse.
Do you drink tea? Do you swim in a pool? Exposure to bromine, chlorine and fluoride (tea plant) will affect your thyroid hormones over time. These all compete for the same receptors as Iodine in your body, thereby replacing iodine, a necessary nutrient for many bodily functions. This was my problem, and now my congestion is almost gone. Search for "Tea, Fluoride and Iodine" to read my story.
Blood tests for TSH may appear normal even when there is a thyroid issue. The only way to be sure is to get the T4 and Free T3 checked as well. Even then, these values do not tell the whole story. Ideally, you can find a really really old doctor who still remembers how to use the ankle reflex test and other clinical tests for thyroid, not just blood tests. High cholesterol is another indicator of thyroid issues that is not well known.