My turning point was finally (after several months) hooking the PS3 upto the Internet and discovering seasonal events!
I did my licence stuff and soldiered on through the game, earning tiny sums of money to buy a car for a race, spend ages doing all the races I could with it then having to sell it for 1/4 what I paid for it to buy the next one...... Now I know that's kinda the pint but I like having lots of different cars so realising that seasonal events could give you SOOOOO much in terms of credits and prize cars was amazing. Took little time at all to progress then!
Having enough credit meant I could easily toy with tuning parts and different cars to get better, rather than end up stuck with a rubbish car and slows progress.
I just never bothered with it BEOFRE as I had no intention on racing online or anything but blimey I wish I had earlier!!
The Japanese 90's races have given me the most in one go Id say too, as got the 787B and earned over 500,000 credits per race won. Really helps you build a garage and make sure you're competitive.