Try this on for size- You said you came from Subaru, which has all wheel drive, and far less horsepower and probably weighs close to the mustang. Let's face facts here. A Forester isn't built to be a "sports" car. As such, it's gearing isn't aimed towards acceleration, It's aimed at getting torque down on the ground. Also, due to the fact it has less horse power, you've trained yourself to expect a certain output from a given amount of throttle input- i.e. you expect at half throttle to climb in speed at a known rate (X per period of time). Due to the fact that you're used to having all wheel drive, you're NOT used to power ( however small it may have been) being delivered to ONLY the rear wheels, meaning your contact patch available for power delivery is effectively half of what your used too having.
In short, you're asking less contact patch than you're used too, to apply more power than you're used too, in the same amount of time that you're used too. The vehicle dynamics are vastly different between the two cars, and you simply can't drive them in the same manner.
You need to re-learn how to drive in THIS car in order to be as effective behind the wheel as you were in your old Forester. Be safe, and approach your limits and your car's limits slowly.