Main thing is that neither you nor anyone else was hurt - just a huge dent in your pride and very understandable given your car history and dedication to learning to drive fast and well on track. And there is no doubt IMO, and in my experience, that skills you learn driving on the limit in controlled conditions does help you understand how a car behaves and know your own ability, something that should not be under-estimated but also serves to ensure you are a more informed driver on the road. But also I would expect 99% of our members are on this forum because driving is more than an A to B exercise - sadly something for the vast majority of drivers out there is their only focus and who take no interest in learning more after passing their test.
Back to your situation Dan, I have to ask the question. Unless I missed it you do not say what the tyre situation was on the car - something very dear to your heart. My guess is that they were the right tyre for the car and in A1 condition...............? Were all 4 fully inflated after the accident and did you check the pressures to see if they were as they should be afterwards? As Mr Lizard mentioned, just maybe, if the tyres were all OK, one of the driving wheels may have caught something on the surface enough for it to spin up but then only you know whether that car was bereft of driver aids (TVR style), or otherwise to help those circumstances.
Had you been pushing on or it was chucking it down you would have been 100% focused on the slide potential so whilst you are kicking yourself for letting the car get away from you it sounds like it was a more relaxed outing that on any other day would not have produced dramas.
Take heart from a friend of mine who during an interview for the post as a journalist on a leading performance car magazine was asked whether he had ever had a bad accident in a car and having come clean and said he had written one off then found he was offered the job with the advice that he will then know what it is like to experience an accident, which they had found made their road testers less likely to repeat.
In time as your confidence returns - and it will, I expect you will actually look back on this as another lesson learnt to make you an even better driver.