Hi there, since you said that your problem occurs during the summer but not in the winter... it would most likely be seasonal allergic rhinitis. Seasonal allergic rhinitis means that the person is allergic to the pollens of some kind of plants. It can be determine which kind you are allergic to by doing a skin p-r-i-c-k test. The blocking of the nasal passages is because your turbinates swell up, thus the blockage. Nasal steroid sprays should be able to help with the swellings and it is said that nasal steroid sprays are often the best way to treat this kind of symptoms. However, I'm one of those that do not get effective results from it.
Nasal decongestant would help open up the nasal passage way by constricting the blood vessels in the turbinates so it would become smaller. It's effective but it can't be used too often as there're rebound effects. The nasal decongestant can come in a nasal spray or drop form. There's also an oral kind of decongestant but the nasal kind works better. You said you took Claritin D and that has a decongestant and a anti-histamine in it. The anti-histamine is to help with the mucus production so that you won't have runny nose.
Have you seen any specialist yet? What I think you need is a skin p-r-i-c-k test to determine if you are allergic to any allergens like dustmites, pollen, animals, feathers and stuffs like that. If you are not allergic to anything, you are probably having vasomotor rhinitis. Your doctor might order some extra tests like Ige, ELISA, etc if he think it's appropriate. You might opt for it if you want.
I can understand with the dry mouth. Since your nasal passages are blocked, you will tend to breathe thru your mouth to compensate the lack of air... If your problem is during the whole day, then you might want to drink plenty of water daily. If you get dry mouth when you wake up, going to the bathroom to rinse your mouth should help the dryness.
Before I end, don't try to force air into your already blocked nasal passages as you
might do damage. Breathe thru your mouth instead. Drink plenty of water too and sucking on a candy might help (dryness) too. Good luck and let me know how ya been doing...
Hope all this helps...