OK, I've thought it through while in the shower, aka best thinking place, using all the powers vested in me by CBT, and:
– The VERY WORST THING this could mean for 'me as a driver' is that I'm not well enough to drive and will have to continue with public transport for the time being. HOWEVER: a) I drove WELL for 80% of that lesson – this is something I agreed upon with long-suffering instructor. If I'd driven as badly as I did in the 20% of that one lesson for all of my other 15-odd lessons, then maybe I'd have to reconsider, but this was a blip rather than a sign that I'm just generally impaired. And b) Even if a) is not true, not being able to drive is a bit of a first-world problem anyway. I've managed on public transport for 26 years now and it is not the end of the world.
– I didn't damage the vehicle, and even if I had, I know people who've been driving for years and who chip paintwork on a fairly common basis *coughmydadforexamplecough*. I'd been upset that I'd stalled a few minutes earlier, it affected my concentration as I went round the bend, and for next time, I know the importance of making sure I'm OK, mental-wise, before driving off again, thus avoiding carelessness. Taking a corner badly isn't something that only the very stupidest of drivers do, and I'm a long way off even taking my test.
– While I can't blame it for EVERYTHING (much as I'd like to), I have an illness that makes getting out of bed sometimes seem like the hardest thing in the world. It was only a couple of months ago that I wasn't well enough to work. And while the final terrifying 15 minutes of my driving lesson will obviously have made more an impression in my mind than all the other minutes and hours that were fine, I do have to focus on the stuff that went well, because it's no good to anyone if I obsess over the bad stuff.
– I've been through way scarier things just in the last month or so than me driving badly for 15 minutes (don't know if the same can be said for Traumatised Instructor) and can bounce back from this just like a goat on Goat Simulator.
– I'm about to watch the next episode in the fourth series of Game of Thrones, so can happily retreat into that violent world (in which cars do not exist) for a bit.

SEE LOOK I CAN DO MENTALLY NORMAL? I THINK? Or at least 'quite mad but in a good way'. Please to praise for turning brain the right way round again.
Now I am to settle down with tea and biscuits, for I recently turned 86. Tomorrow evening I shall meet up with Lord Richington, who is wise enough to visit Alnwick! And we shall frolic. Then on Thursday I'm back down to the Epic Midlands again to troll my family and maybe force my dad to tell me of stuff he's done while driving so I feel a bit less bad.