My 2 cents, as a "real" (PPL) pilot and as a flight sim guy:
Don't fly the big airplanes with lots of sophistication. Fly something small, single engine piston, until you fully understand the relationship between power, pitch, speed, trim, etc. (The secret is, for any particular configuration, trim sets your speed. When you let go, the airplane will change pitch until it finds the same speed again.)
From your description about speed changes, you don't fully understand the need to adjust pitch/power when you change your configuration. This is something small airplanes can teach you better than big airplanes.
Once you fully, 100% understand that, then fly the big stuff. I don't fly 777, but based on your description, you just need to FLY THE AIRPLANE. If you're on G/S and stable, when you change config, don't mess with the trim. Adjust power and pitch to get the airspeed you want while still on the G/S. Only after that, trim to remove the stick-force. (If you knew how much you need to trim before that, you can do it at the same time, but only when you have the experience.) The trim is not a primary flight control, it is there to neutralize the stick in stable flight. Don't try to fly the airplane with the trim as a primary flight control.