Are you getting beaten badly on the boards? Try 2-3 zone.
Actually, I would suggest a move to man-to-man for a few games and see if that improves your D at all. From there you can either tighten it up with a 2-3 if you are still getting beaten inside, or you can stretch it out to 3-2 or FCP.
It is important to change your tactics occasionally or else your opponent will be able to dial in a tactic that works well against your standard schemes.
If you want to play FCP you need to improve your stamina and OD. Since your good players are guards, this would also increase their value on the TL os you can uograde down the line.
Good luck.
Once I scored a basket that still makes me laugh.