Pinkflyer,
It's probably true to some extent that some are born with more capacity than others, but I don't know if you can really "improve" your capacity.
You can certainly free up capacity once you get more confident in handling your aircraft and have to devote less time and mental effort to just keeping the damn thing airborne! Also, the more currency you have helps a lot in my opinion.
Congrats on wanting to go jets, keep working hard and aim to be the best you can, that's all anyone will ask of you, and all that you can do. Good luck!