Your lungs are telling you they don't like what you're putting in there...at all! Since it's been 6 months of coughing, that should be pretty clear by now.
IMO, if you're sure you don't want to give up on vaping though, because you need the nicotine...I'd stop inhaling the vapor into your lungs! Try bringing it in only as far as the back of your throat...wait a couple seconds, and then exhale it through your nose/mouth.
I smoked 1 - 1 1/2 packs a day for 42+ years and inhaled my ciggy smoke all those years. However, I find that I definitely get the nic I need, from my 6mg eliquids...by only vaping the way I said above! Nicotine in eliquid is readily absorbed by your cheek cells, as well as the mucuous membranes in your nose and sinus cavities. In fact, when I first started vaping, I was told by a longtime vaper that absorption was better that way, than inhaling it into my lungs. I don't have any scientific proof of that, but it has worked for me and many others who vape that way. I've never felt nic deprived, nor wanted a smoke, by vaping like that for more than 3 years now.
Might be worth a try for you, too. I'd also recommend that in addition to trying the different inhaling technique...adjusting your PG/VG ratio so you're not at 100% VG. That 100% VG alone can cause your lungs to feel pretty "heavy". If that doesn't work either, personally, I'd pay attention to my lungs and quit vaping.