I can't help much because I suck. But I have a DFGT wheel like you and have been trying to get better.
http://www.virtualracersedge.com/physics_of_racing.htm : stuff about physics of racing
http://www.race2play.com/homepage/show_blog_posts/333 : a guide to sim races
Those have helped me. I don't know how much, but I'm sure they have. You may also want to find the Gran Turismo driving guide from the instruction booklet of GT 2 (I think it was GT 2, could've been 1, I don't know). It's a .pdf that's really, really easy to find on Google. It has got some interesting tips.
Pick a small amount of cars that you like which are not completely similar to one another. I like the RX7, any GTR, and the California, for example. Practice with those. It might be best to pick just one, but that can be boring after a while.
Do time trials. You'll have an easier and better time improving without worrying about others on the track. Plus you'll have nobody to bounce off of in order to save yourself. It can be frustrating to keep doing laps alone and not be able to reach the time you just set the last session (a feeling I'm having as I write this after a session on Fuji) but it's such a good barometer of your improvement to track lap times this way. In a month and a half or so I've gotten about 5 seconds faster on Fuji (from newb to semi-newb).
It's difficult and annoying to get better but it's like that to get better at anything, so keep plugging away!