I would recommend several things:
1) a better antihistamine. Benadryl is very weak and high in side effects. Zyrtec and Allegra are both OTC and generic and work great.
2) sinus rinse!!! My all time favorite addition to my life since my allergies demanded treatment. I prefer the NielMed sinus rinse bottle (it is the only bottle available) over neti pots as you can get a slightly stronger flow to better cleanse your sinuses of debris and pollen. This should be done at least twice a day, but can be done up to 5 times a day if you get an infection or cold.
3) Allergen avoidance: shower and change clothes when you come in from long periods outside, shower before bed, wash bedding every week in hot water, keep pets out of your bedroom, allergy proof covers for pillows, matress and boxsprins (mainly for dust/dust mite allergy), keep doors and windows closed as much as possible.
4) consider seeing an allergist, being tested and starting shots. They really work and do wonders for control after 6 months to a year.
There are many other options a dr or allergist can discus with you like nasal steroid sprays or nasal antihistamine sprays. They work very well.
Feel better.
You need to get evaluated clinically as well as certain tests will be help to determine the cause. You also may need to undergo surgery for deviated nasal septum. This can relieve recurrent attacks of sinusitis and secondary infection.
You need to continue with Anti histaminic against prescription. Sometimes nasal sprays of corticosteroids are useful. Remember, symptoms can be controlled through medication, but you can't get rid of the allergy itself. Close the windows when you sleep. I suggest you to consult ENT specialist. Take care and regards.