Hi, U33 last year about this time I travelled on an A 380 top deck at the back (Emirates) and you feel more like being in a large room. Very little sensation of anything really, even take off and landing. In a large aircraft there are fewer sudden movements.
I always go for an aisle seat, I flew about 45,000 miles last year, a record for me. If you select an aisle seat in the 3 seat configuration you frequently have an empty seat next to you.
Most long haul flights will need you to decide where you want to sit on-line 24 hours before you are due to fly. Some times if you book earlier you may have to pay £25 per flight! But when you have booked / paid for your seat there should be a seat allocated to you. It is just doing it for the first time that is daunting!
If you go one month and come back the next month you should get a change of films etc.
As far as travel insurance goes I have a current account that gives free insurance for EU travel and cost £20 to upgrade to World wide cover for a whole year. But as Neil says don't go without it.
Worrying about 7 miles of 'thin' air under you is as daft as thinking about how deep the ocean is when in a ship.
We all have to live with a degree of trust placed in others especially in the transport business. I have a cousin now retired who drove an underground train in London, now you could reall worry about being in a tube train! But I do it on a regular basis and I am claustophobic and sometimes have a bad night and have to keep the light on.
Not easy but a bit of Displacement Therapy helps, take a book or plan you next model boat etc.
So a bit of common sense plus checking on what your fears really are, it is not flying which is the problem it is crashing and at least you know the pilot is trained ( I like to find a lady pilot as they have had to work harder to get the job!) not so a lot of the car drivers you will meet on the way to the airport.
Good luck anyway check what is the worst that can happen, see how often it happens and then compare it with driving on our roads.
A small tip, we all take water with us now but you cannot take it through security. The solution is simple ,empty the bottle before security and take the empty bottle with you. When through security you will find outside all of the toilet areas there are are cool water fountains and fill the bottle there.
While flying last year I saw a film I would otherwise have missed "Robot and Frank" it's a great film, should be on TV in a year or so don't miss it!
good luck and kind regards Roy