Just keep going back to the doctor until you find some hayfever meds that work on you. I've been through cetirizine hydrochloride, loratodine, piriton, a bunch of medicines I don't remember the names of and right now the one that is working for me is fexofenadine hydrochloride and my hayfever is extremely bad like itchy, stinging red eyes and stuffy, tickly nose and loads of sneezing and eye watering and overall feeling horrible. I know how you feel about being in the exam, all I've been doing in exams is blowing my nose and sneezing loads.
Make sure to go to the doctor and see if he/she can prescribe you something stronger or more suited to you
Also
-having a hot or cold bath will make you feel refreshed and usually opens up your nose and just helps
-washing face with cold water is extremely refreshing and make sure not to rub your face too much with a towel or when you're washing with facial wash or soap because it can make your eyes more irritated and raw
-using a vicks nose inhaler or just sniffing a tub of vicks opens up your nose if it's blocked or stuffy and sometimes just kind of speeds up the process of sneezes in a go so that you feel better afterwards and your nose doesn't feel all tickled and itchy