I’ve recently returned to european driving after a 7 year absense,but have encounted a problem.I have always had a fear of heights but never had problems when driving.The 1st incident was the bridge on the ring road at Nantes,this was followed last week by the terrifying viaducts going towards Genevre in France,i was petrified i slowed down to about 30mph and just gripped the steering wheel as tight as i could until i was over them.It doesnt help of course that you are dring a rhd truck there is no hard shoulder and a measly 2 ft high fence which would stop sod all.Any body have similar problems or solutions any help would be greatly appreciated.
I am the same.Even going over Dartford,I try to stay in lane 2 or 3.Going up to Mont Blanc on french side scares me. 
Yet I have stood on the roof of Canary Wharf and looked over the side and that doesn’t bother me.I think its just the thought that the barriars on bridges are not strong enough.
I am not alone!!!
I used to go out with my old man when I was little in his truck and started having a recurring nightmare about being blown off a high bridge and still get slightly nervous to this day. You sound like you have it way worse than me. Have you thought of hypnosis or NLP, because it’s a fear the same as a fear of flying and of spiders and that can help.
Try working for a company where you are working at heights and be scared of heights?
Got through that by trusting and relying on the safety systems, the harnesses and the like. Never did my HGV as they’d send me out on the bigger vehicles (ie taller platforms) but now regret not getting my HGV then. Hey ho.
Concentrate on the job, check your mirrors, look at the road and the supports, concentrate on the road and your mirrors for the road and ignore the terror ripping through you, bringing you out in a cold sweat and bringing your heartbeat up.
Good luck. It needs a bit of conditioning but I’m sure with a bit of care you can concentrate on the road that you know and get through it…
You need to take up a relaxing pastime and enjoy it. Have you tried swimming?
I have the same roblem with going over dartford, I just concentrate on the vehicle in front and try not to grip the steering wheel too tight as I find this makes the truck become a bit jerky. I also try and think bout comforting stuff and before I know it I’m over the bridge. My mate uses the same method and it works for him too. Happy trucking 
I came back from Mt Blanc again this week over “those” viaducts-i think as time goes by it does get easier sometimes making an effort to look down does help.