That is strange man. Are you playing world tour? You don't get to set difficulty on that mode.
I'm on the Forza forums and most people are complaining about how hard it is.
Its not about the event or the car type. Its about the fundamental way the cars drive.
Have you played the previous games? The very first, on Xbox, was super evil hard... but an amazing rush when you hook it up. It has gotten progressively easier to drive with each new version, but the jump from 3 to 4 is absurd.