I asked same question 2 months ago.
This is how I got better. (PS3)
1st I bought racing wheel ( I got DFGT)
2nd I tried to search a lot of settings and hoped that they will help me. But really settings help u improve time, but settings dosent make u good driver. So try to drive with default settings for while.
3rd Learn how to drive with manual shift. That improved my driving by 100%. U can easier manages speed in corners, u can brake late, u can do a lot of other moves if u drive with manual shifts.
4th Try to accomplish ALL TROPHIES! That will push you to make restarts, change tracks and spend a lot of time learning those tracks. Also try to watch Youtube videos how to make better lap times in Time attack mode etc.
5th Dont run @ Online mode in early start of game. Do all trophies, do all tracks and learn everything from game before spend a lot of time in Online sessions waitings players and then rage quit on them.
6th Turn off ALL ASSISTS except racing line. For start get used to braking without ABS and other assst help. But I dont think that you should turn on racing line. Becouse that line help you also adopt to manual gear shifting.
Time: I spend 8h in 2 days @ Melbourne learning how to use manual gear (I use manual + suggested)! And then I started to drive Time Attack challenges, Time trials and also Season Challenges.
I did almost all tophies in 4 days and I played like 8-10h in day. After 1 week I was pretty much normal driver for Online. But I still drive with racing line.