Got Fanatec gear recently, having a lot of trouble adjusting. Anyone have any tips?

The difference between playing with a wheel and a controller is night and day. I received my set last week, including the CSR wheel, CSR pedals and Porsche Shifter set. I have it hard-mounted onto a pretty stable desk, although the shifter is still necessitating a lot of support. I'm having a really hard time getting used to it. I've set it to 900 degrees of rotation, ABS on at 100 and dampener -3 like it said on the web site, but factory settings otherwise. Controlling oversteer is a mystery, and when I'm exiting a corner,as I start straightening out, the car will fish tail back and forth endlessly until I brake or the wall brakes for me. Downshifting and engine braking (sans the engine breaking hehe) sends the car out of control, even with most driver assists on (TCS, ABS, even STM) and makes it hard to make good lap times or make laps at all. I know how to drive stick IRL, just not as aggressively as you have to while racing.

Now that I've ranted, I just want to get together some tips, specifically on controlling oversteer, and the chaotic fishtailing. Hopefully this will solve more than just my problems and someone else will find use your knowledge.

EDIT: Some info I didn't think to add at the time, I'm playing FM4, and I do try playing with as few assists as possible. I was use sim steering too. Thank you guys for the feedback and I'll get around to trying out these different settings, experimenting with them as well.