Nobody else really knows where to focus their study either so welcome to the club. Now if you don't mind spending some serious cash there are any number of good coaches out there. That's what they do, figure out your weakest area and tell you how to improve. As Martin pointed out, you really can't go wrong with tactics, but even that is controversial. Coach Heisman thinks one of the reasons that players plateau is because they never spend the time necessary to recognize basic patterns. Sure they can figure out problems but that's not the same as recognizing the solution because you've committed that pattern to memory. Sorry if this is old hat to you, but Coach's well known metaphor is the difference between addition and multiplication. You actually could stumble through life adding 7 to itself 8 times, but knowing 7x8=56 makes things easier. So maybe set your tactics trainer to unrated and a couple hundred points lower than it is currently. When you can consistently solve 85% or more of the problems, bump it up a little. I do it 10 points at a time. Oh I still do a couple of rated problems a day but most of my time is spent unrated. Let me give you a link to NM Heisman's site where you can read 10 years worth of his monthly suggestions on how to improve.
http://danheisman.home.comcast.net/~danheisman/Articles/Novice_Nook_Links.htm
I think you could say chess study is a "very dark and deep cave" and it will take a lot of work find the materials best suited to you and your skill level.
You might have a look at the Study Plans.
Tactial study is always a good thing for pretty much anyone. You need to become familiar with a lot of patterns and most games at lower ratings are won or lost on missed tactics. Tactics Trainer here is pretty decent for that, though there are ton of other resources out there too.
Finally, Chess Mentor is pretty good resource. I actually set the mode to adaptive, with the lesson pool being anything where I haven't had a score of 95% or greater. Many of the lessons there go into more detail than what you might get in just tactical training.
Hopefully that will give you some ideas. Also, keep playing some longer time control games to try and practice the things you are studying.