cough! That won't go away!?

I've had a cough since start of September it wont go away!. It's starting to get me down now!. Sometimes it goes and isn't as bad. but then comes back and i have it most of the night?. I've been to the doctors 3 times about it. I've had astma test all clear, x ray all clear. He's now put it down to my hayfever?. I constantly have a cold and sneeze all the time. it feels like something is annoying my throat!. I'm 26 i've never smoked!. i had the same problem last year but as soon as i went on holiday and weather was hot the cough went!. I was worried i had lung cancer i havent lost weight and i'm not tired or coughing blood. does anyone know what i could do or take to get rid of it! I've tried nasal sprays they only help a little!