Tercel, some of the tracks in GT3 can take a lot of getting used to and Rome is particularly cruel to heavy, high horse power FR cars (it can feel like the roads made of polished glass!).
If you've not practiced on the circuit all that much, it might be an idea to spend an hour or so on it in the Arcade Free Run section. That way you can refine your lines and braking points without being hassled by AI cars.
By coincidence, I've just rerun this championship tonight (as a result of a tuning thread on another forum) in an Mazda MX-5 (J). I was disappointed how easy it was in that I won each race easily (4 or 5 second lead) despite the fact that there were cars like Zonda C12's, Griffith 500's etc in it. I think they can't be modded in any significant way as there is no way on God's Green Earth that a little MX-5 should be able to out-accelerate a Zonda!
My real point is I guess that you might be better off with a lighter, more nimble, less powerful car. But if you're stuck for car choice or have got your heart set on using the Merc (lovely executive smorgsmobile I have to admit) try posting the mods you've done to the CLK and ask for a bit of tuning advice. There's no shame in it and you can learn a lot from seeing how other people set up their cars.