You are stalling out from turning to hard, loosen up and "lag turn" behind them.
wider turns conserver energy and speed.
tight turns bleed energy and leave you clumsy.
If you feal like everyone is "out turning" you it is because they are. In saying that i dont mean they can turn tighter than you or more efficant, they are turning smarter than you. If someone is in a sustained flat turn for example with you co-energy behind them you want to make a slightly wider turn then they are to persue, its called lag-turning. My genernal rule to lag persuit is to keep the enemy a good distance above your aim point. Dont try to constantly keep them in your sights, you are lead turning them if you do and loosing speed(energy) drasticly
This is effective becuase in a lag turn you are pulling less G's than the enemy making your plane of flight a wider turn. If he stays in a sustained turn he is "meat on the table" becuase in the end you will have a SERIOUS energy(speed) and angle advantage over him, thus more manueverbility.
Best way to understand this is to draw it on a peice of paper. draw two points of refernce following the same line, make one (lead)curve in sharp and the persuit curve at the same point much looser both in sustained manners pay attention to where and how (angle) the planes of flight intersect and remeber at the point the lead "plane" will be much more sluggish.
im sure you get the point
good luck!