Well, it sounds like the problem is.... you're "shit".

That will take time to fix. Experience is so important. But strategies and tips will no doubt help you improve faster.
How long have you been playing the series for?
xabre said:
What must I do to be at least competitive online (Im no pro offline either for that matter)? What are some strategies for organising my defence so that it is not so susceptible to through balls or splitting wide open all so often? And what are some strategies for sound offensive build up?
On defense, guarding against through balls:
- you should pretty much always have someone in front of the person carrying the ball. do not lunge at them (not always, i mean), but jockey. holding R2 will keep the defender positioned well, but naturally hurts quickness (so, use a combination of regular running and R2 jockying). this will cut off passing lanes (and prevent them from dribbling easily, of course)
- do not constantly pull defenders out of position. be conscious of where each player belongs and what his role is. eg, do not have a CB follow a forward around the pitch, as it will create a hole in his area of the field. switching players appropriately is critical (and will take time to do properly) **this should help you against being split wide open
- sometimes the ball carrier will be in a situation where he can give a through pass easily (no defender marking him). in that case, be aware of the forwards who are making runs, or potentially could. maybe pull your defender backwards so if a through pass is made, you can either intercept it or be on the other side of the forward who receives it.
Sound offensive build up? ...where to begin? It depends on your team, formation, and personal style of play. It's probably best to observe your opposition when playing against them -- I know you'll be busy worrying about defending properly, but actually understanding how they're successful will definitely help you on offense.
Otherwise, I don't know what advice I can give you. Try the Training Mode (it's very good). And a few pointers....
- L1+X is important... especially when you become comfortable with using it as a one-time pass (ie, pass to player, press L1+X before ball reaches him, said player first-touch passes then runs)
- remember that through passes (triangle) are pressure sensitive and will go in the direction that you press.
- move the ball forward with your DMF/SMFs -- try beating your opponent one on one (if dribble accuracy is high, i mean) and using little give-and-go passes between your mid-fielders. this will provide you with greater odds and more choices when you get upfield. otherwise, if you just immediately move it up to your forwards, they'll have difficulties. in that case, never be afraid to pass the ball backwards. even all the way back to your defensemen. control the field.
I've got to stop myself. There's more I could say (re, crossing, etc), but I'll leave it to other people