How do i set up a good defense?

I'm struggling with my defense, i play a 4-2-3-1 formation, generally i create a lot of chances every match, and score 1-2 goals at least for most matches, but i also never keep a clean sheet, my defense sucks! and i don't know what to do about it, my defenders are slow, so i have instructed them to drop deep and my full backs are set on support, i'm currently on top of my league, but a overachieving a bit.
most of my victories are typically 3-2, 4-3, 5-4. You get the picture..
I know it is difficult to belt when you don't know, all the details of my team, but the general problem is slow defenders with low intelligence, i'm playing in the skrill premier so, so basically all the defenders i can get are slow unintelligent idiots, with bad heading and tackling skills.

Hope you can help,
Thanks

I don't think there is an easy solution to your problem, as setting up a good defence will likely dilute the attacking nature of your side. What defensive options do you have checked on your Team Instructions? Do you have any particular instructions given to individual players? Are the '2' in your formation central midfielders? How do you have them set up? Basically, if you described your tactic in more detail it would be easier to make suggestions to help you.

Central defenders are instructed as limited defenders, since they have no "attacking" skills, full backs on support, 2 MC's ball winning (D) and box to box (S).
2 wingers (S) AM Advanced playmaker, and striker as complete forward.
Team instructions are set to Control - Balanced.
Drop deep and stand off opponents
Short passing, retain possesion, run at defense, drill crosses, pass into space and be more expressive.

Defending are not only denfender's duty, maybe you need to ask your midfielders to help~

what is your overall team philosophy? I've found (especially in the lower leagues) a defensive philosophy works well and doesn't limit your team scoring goals as much as you'd think.