Wow, this is a great example as to how different players have a different experience. I do endurance karting whenever I get the chance, and I was just about to start a thread griping and whining and moaning about how the Karts in GT6 are way, way, way too easy to control! In most (real) Karts just breathing hard on the brake pedal in anything other than a straight line will send you into either a spin or (if you are good), an aggressive "drift" with the rear while you counter-steer like crazy (because most Karts have only rear brakes). In GT6 I can slam the brakes all the way down no problem during any turn I want, and as long as I don't leave the pedal down for more than like 2 or 3 seconds everything is fine. That's FAR more forgiving than any real-life Kart I've ever driven (in a real Kart, just hitting that pedal for a fraction of a second can send you into an instant uncontrollable spin until you have a LOT of karting miles under your belt). Heck, even braking in a straight-line in a Kart can catch you out if you aren't careful. As twitchy as the steering is in GT6, even that's not anywhere near as tricky as steering a real kart.
Anyway, I think the upshot is that if you are having trouble with the Karts in GT6 go out and enter a local "open" Kart enduro event with rental karts. I'll probably cost you around $150 unless you can get a big group discount. Learn to drive the real karts and then come back to GT6 and you'll find the karting there to be incredibly easy. You'll also have an absolute blast driving real Karts. 50 or 60 mph feels a LOT faster when you are sitting that close to the ground...