Must be very difficult for you, I hope someone can help you.
You seem to have a classic allergy, - though you would need to get it checked.
for allergy tests there is a skin prick test, quite good, they test you for different food, moulds, plants, animals.
You can also get a RAST test, it's a simple blood test for allergies, but it's not as efficient as the skin prick one and could cost you more. Depends how much the government pays.
Lots of people who have allergies changed their diet.
You could avoid milk and dairy products for a while and see if that makes any difference
as for allergie treatments:
saline sprays, it's easy, and might help a little bit
antihistamine tablets - if you have mucus, runny nose
If you use neti pot, the water needs to be luke warm. If it's cold it might just irritate you even more.
If the allergy test comes back negative, it doesn't necessairly mean you have no allergies, it might just be that they didn't find it.
You mentioned that sometimes your nose is completely stuffed up.
You can't use the decongestant sprays - it's max for 3 days, using it longer and you will get a rebound effect
But you can use pseudoeffedrine tablets, don't use it in the evening, you might not be able to sleep. But durung the day should be ok. Don;t use it all the time though!
The next best to pseudoeffedrine, you can order in form of tea. It's very mild, I tried it and have no problem sleeping, quite safe. PM me if you're interested and let you know how to order.
That's about how much you can do yourself.
ENT can do further checks: polyps, deviated septum, turbinates, anything, which could physically block your sinuses.
Keep in mind they might not be able to find the cause of your sinus problem either, sometimes it's some unknown allergy and unfortunately not much research has been put into this area.