Sorry to hear this Dan.
Firstly, I like your new signature!!!
Secondly, well done for taking it on the chin and admitting it's down to you.
Thirdly, you have always appeared to me to be most particular and thorough about understanding and gaining as much information as you can about subjects that interest you, tyres and road handling to name two that I am aware of.
And last but not least you have had an almighty kick in the "confidence" teeth because of the following contributory factors (all of which you seem to agree with).
1 Your mate trusted you and your ability enough to lend you his car and you trashed it (hugely embarrassing).
2 This was your first accident and it happened in your mates' car on a road that you are familiar with.
3 Having never had an accident before and being seen in the eyes of your mates as being on a "bit of a pedestal" in the wet you have to deal with the fact that you aren't invincible after all, and worse still, in your mind, you fecked up.
Given all that you know about tracking, performance cars, tyres, road handling etc I would quite honestly have to say that you have been tremendously unlucky for the following reasons:-
1 Diesel on your tyres?
2 Diesel on the road?
3 Damp road
4 Change in road surface probably very close to the point you accelerated.
There is more than a fair chance that at least 2 of the above contributed to your mishap.
Combine all of these possibilities with the way you feel now and is it any wonder you're taking it all to heart AND suffering a loss of confidence? Your pride has been severely dented and like the car it wasn't write off but it does needs reparing.
Seems to me you need to get "back on the horse" as soon as you can, you don't forget all the things you have learnt in one split second, you need to reassure yourself of that ASAP and put it all down part of life's rich tapestry.
Your mate has forgiven you, now you need to do the same, forgive yourself and stop fretting about it.
Well that's my twopenny worth.