How long has it been since playing the outfield? Have you played outfield on the big diamond before? It's easy to lose your feel for it.
Playing outfield in LL is over simplified since you really don't have to judge the ball. It's either in front of you, or it's a home run.
On a full-sized field, in general, it's easier to correct IN than BACK. Always go back hard keeping your eye on the ball until you are sure you have outrun the ball. You can also tell as the ball significantly slows down. Now adjust your movements to settle just behind the ball in line with your intended throw so you can step into the catch and throw it in one fluid motion.
The key with most skills is understanding skills are perishable and getting sufficient quality repetitions. Get out there and shaggy some flies. Start out in center because flies are easier to read from center. Then try the corner positions next.