Hi - I am an expert on this topic - had it for 30 years and after many tests and operations found out that it was a combination of acid reflux into my throat and dust mite allergy. Your symptoms are typical of acid reflux into your throat, but also could be exacerbated by a dust mite allergy. Although you can try proton-pump inhibitors such as Nexium for the acid reflux they didn't work for me - I had to have a fundoplication operation - then to stop the cough/throat clearing completely I had to have sublingual desensitisation for my dust mite allergy. Go and talk to your doctor and ask if you can try a proton pump inhibitor - they take at least 8 weeks before they work (if they are going to work). If this isn't succesful - go and get some tests - a scratch test for allergies to different things and a 24 ph monitoring for acid reflux.
Good luck and persevere
I was having the same problem recently, without cough, just excessive mucus build up in my throat. It was worst in the mornings. I thought I would drown in mucus, nothing seemed to help, allergy meds, decongestants, nose sprays, nothing worked.
Then several days ago, I started Nasal irrigation, 3 times a day, rinsing out my nose with saline, the mucus production is almost completely gone, I only clear my throat 2-3 times a day now rather then a zillion, Lol,. Maybe it will help your symptoms, you can buy the saline nasal rinse almost anywhere.