To the OP: I completely understand how you're feeling. I've recently had the onset of virtually the same feelings. I was due to go on a mate's stag do to Tallinn in Estonia at the end of March. Almost without warning about 10 days before the flight I started getting nervous and negative thoughts which grew & grew to the point where I wasn't sleeping, hardly eating & couldn't concentrate on anything at work or at home.
I went to the doctor's the day before the flight to talk about it & he gave me some Valium, including one to 'try' on that day to see if it helped at all, which it did a little - but only to the point of getting 2hrs sleep. I ended up calling my mate the night before the flight (flight was at 6am) and telling him that I couldn't bring myself to do it.
Just some of the thoughts that went through my mind were pretty horrible. For example, I had the feeling that if I got on the plane it would crash & it'd somehow be 'my' fault (killing my mates etc.), that it'd be 'just my luck' after 1000s of safe flights by the Worlds' airlines that it'd happen on my flight etc.
The weird thing is that I have flown previously - flights to Spain, Majorca, Canaries, a Transatlantic trip to DC and most recently to Munich in 2006, just not very often (once every 2 years or so) & I've never been what you'd call a 'great' flyer. Also, I'm a pretty big aviation enthusiast - I go to airshows/displays and enjouy reading about (mostly military) aviation. I'm not sure if it is a case of too much knowledge about aircraft & their systems is a dangerous thing - knowing all the things that can potentially go wrong.
I guess at the nub of it are several things: a fear of being in an enclosed space (like claustrophobia), a feeling of not being in control and having to sit there and stew about things in my mind (I'm not very good at all at not thinking about things and have a hard time relaxing), a lack of trust in others' abilities (ground engineers, designers, pilots etc), and ultimately being in an aluminium tin can stuffed with fuel at 31,000 feet..
I'll be honest and say I still think about it quite often, like most things there are good days when I don't think about it & bad days when it's hard to shake. It can be hard to have the 'flying's great, I love it' fraternity not understanding where you're coming from & that somehow you're a leper for not willingly jumping on a plane every 5 mins
Sorry to ramble on, just my 2p's worth