I don't what you mean by "trim the plane",
Then you don't know much about planes
All aircraft need to be trimmed for the airspeed you want to fly them at.
if you mean constantly pulling my joystick back
Check the control list for trim buttons. Use them, and you'll see how it works.
then this is the only Flightsim i know that you have to do that. In all other sims i've played (Strike Fighters Series, Falcon 4 and Free Falcon 5 etc.) the plane doesn't slowly pitch down because you change the speed.
Pretty irrelevant. SF doesn't have good flight dynamics, and The F-16 is an FBW aircraft which is simulated as requiring no trimming at all. In reality, it needs -some-, but not as much as an aircraft that does not have an automatic trim function.
The only FC2 flyable aircraft that has such a function is the F-15C. The rest you must trim yourself.