Mr Joshua (I had a teacher called that but you're too young to be him!)
From my experience of seeing other passengers if you can fly in a 737 around Europe like you have done then you are not afraid of flying, do not panic about long haul, there are some people who would never have flown the return leg of their first flight because they're so scared of flying.
The problem you get with long haul is boredom and morale dropping as you get nearer to landing, then frustration when you can see the airport but its still ages until you're out of the aircraft. Flying to the US watching Scotland, Iceland and Greenland keeps me busy for some of the flight, flying back I'm normally so tired from my holiday that after a few glasses of wine I'm asleep for 6+ hours.
If you're worried about getting frustrated, wait until you get to the US Border Control, that'll make the flying part of your journey seem like a 100m sprint!