Hi. I know just how you feel. I experienced really bad clear air turbulence over the Atlantic, which came out of nowhere. The plan dropped out the sky and the trolleys lifted off the floor, air stewardesses etc. After that experience, I've been really terrified of cruising at altitude (although statistically that is the safest part of the journey).
I did a Virgin fear of flying course, which was helpful, but it didn't actually involve a flight. I think the Aviatours one ends in a flight where the pilot explains every noise etc, so is probably better (but more expensive).
I also did several sessions of hypnotherapy, which I thought didn't really make much difference at the time, but actually probably has had the longest effect. He showed me how to visualise myself in a safe place during any stress. So for example, imagine yourself on the ground watching the plane or floating alongside the plane.
It also helped to think about what exactly you are afraid of (apart from the obvious of the plane crashing!) Some people might be a bit claustrophobic, others don't like the sensation of not being in control. I didn't like the idea of making a fool of myself. I felt like I was going to go crazy with fear, hyperventilate, be sick etc and have to be restrained.
As far as medication goes, I have been prescribed lots of types of medication including valium etc. I personally didn't find it worked, because although it made me physically calm ... I was still terrified inside and mentally felt like I was going to explode. It doesn't stop any of the feelings of terror, but all your co-passengers will think you're much better! My husband certainly thought it worked, but I didn't at all.
I really recommend Bach's Rescue Remedy - don't know how it works and my doctor said it was mumbo-jumbo. Somehow it does work. I put a few drops in a water bottle and sipped it before I went on the plane. And then when turbulence hits, I take LOADS!
Do tell them when you check in that you're a nervous flyer and again when they come around at the beginning of the flight. I find that air stewardesses can then give you reassuring smiles and look out for you.
Good luck!!!