The best prescription in this case, can only be time.
I would strongly suggest that you look up on information related on plane specifications, tactics and basic/advanced flying methods. Flight sim games, especially combat flight sim games, take a lot of time to 'get used to'.
Please take into account that this advice is not just a "convenient way that veterans brush off newbies"
It really does take a lot of time in bascially even understanding what people tell you, when it comes to the flight-sim genre, and how strong the willingness and motivation for the individual to study and learn about flying determines a lot!
For instance, the phenomenon you experience is called 'stall'. To understand what this is, and how to prevent it, you must also study a bit on basic theory of flight. What causes stall? How can it be avoided?
And then.. as you learn the basics, you quickly realize that some planes cannot follow other planes in turn maneuvers, because they are limited in pure maneuverability. You begin to ask yourself "how can I maneuver to an attack position in a plane that cannot follow the target's turn?" ... and from this inquiry begins your long journey into the world of air combat maneuvers, tactics and situational awareness.
So.. give it some time. Pick up some good books, or browse through the internet! Look for information and history of WWII airplanes and air combat! Relentlessly practice with the 'trial and error' mindset! Do not be discouraged of being shot down!
I used to think myself a pretty adept pilot in the days I was flying with my keyboard, with all those 'cumbersome' 'realistic settings' mumbo jumbo turned off
I used to shoot down zillions of planes everyday. Then I got a stick, started flying in Fighter Ace.. hey, no sweat!
Then I took a step into Aces High version 1.5, and boy, was it a long down hill from there
It took me nearly a month to get my first kill, and many many many more months before I became to understand and fly around.
Don't hesitate to ask questions on the help forums.. many people will try to answer your questions(as long as they aren't the typical "RTFM" material
)