Having suffered allergies most of my life, I know how debilitating they can be!
What helped me was 1000mg vitamin C with 15mg zinc three daily - you can get this from the supermarket as tablets that make up into a fizzy drink (tho' they contain aspartame which isn't all that good) or you can get some of these
. Along with this I'd take a quality multivitamin/multimineral like this
, to support your general health.
Probiotics can be helpful too as allergies often develop after antibiotic use or after gut infections when the gut lining becomes less functional due to non-supportive bacteria taking hold. With this possibility in mind I think it would be a very good idea to absolutely avoid all dairy produce for 3 weeks and see if it makes a difference - this includes milk, cream, butter, yoghurts, cheeses, and anything that contains whey (you need to read labels carefully) and check any white sauces. If at the end of this time there is no difference then you are probably not suffering from milk intolerance which is often linked to rhinitis - do let us know.
The other thing is to eat as many vegetables and fruit as you can - 5 portions a day is a minimum - 8 or 9 is better. A portion is 100gms (a LOT!) and it is not that easy to get through this much - fruit smoothies for breakfast, snacks such as apple, pear, carrot sticks, celery sticks, big salad for lunch (take a lunchbox with you) as it is best to eat about 50% raw), and 3 veggies for supper. This will help to supply the countless micronutrients not available in a vitamin pill that most of us are desperately short of when we live on the current Western diet. Do have a look here
for an overview on the difference diet can make - it can change your life!
In the short term, to relieve blocked nose, do avoid nasal sprays unless absolutely necessary as they have a rebound effect, actually blocking the nose up after a while. Using an old-fashioned method can help - take a bowl half filled with boiling water, add a couple of drops of olbas oil or some vicks paste. Cover your head with a towel while leaning over the bowl and inhale the vapours until the water has cooled (10 minutes). However, I expect the dietary method should help in due course - but not immediately as it has taken many years to develop and may take a while to go.
I would also like to point out that long-term use of antihistamines can suppress the production of stomach acid which in turn can lead to overgrowth of 'bad' bacteria in the bowel and all sorts of unpleasant consequences down the line - see this book
for full details.
It is also possible for something around your house to be causing the problem (animals, bath products, aerosols, carpet treatment products, perfumes - the list is endless! - but I'd try the dietary route first by building up your immune system to react normally again.
Hope this helps, but if not please do see a qualified nutritional therapist
who will be able to help sort out the root cause of your problem.