Nil Flaps,
Let's calm down, and look at what actually happened...
the engine spluttered for a moment. As soon as I heard it, I decided to pull the power, braked, turned myself round and exited via the taxiway.
A sensible decision. Better to sort out these things on the ground. The last thing you want is to find out it was serious when you had engine failure at 300 ft, just after take-off.
A bag of nerves, I taxied back to the school and shut the engine down. Or at least I tried to! I've only ever done this once before, (about lesson 4) but after setting the throttle to maintain 1000RPM, I pulled the mixture full lean to stop the engine. Obviously due to nerves, I didn't wait for the engine to cut out completely before pulling the throttle back to idle
So, you were understandably stressed out, and forgot how to do the shutdown. It happens. Recently, when hovering with a student, we saw smoke coming out of the instrument panel. He landed, and mixed up the order of shutdown. I took control...and I got it wrong too! I've only done it several hundred times. No harm done, but it's worth knowing and recognising what stress can do. That's why we have checklists.
being so rattled after an aborted touch & go, I just couldn't work it out.
Yep, been there. Classic stress reaction. Absolutely normal. Everyone in the universe reacts like that. Well, maybe some super-people don't, but I never met them, outside of movies.
I'd left the master switch on; the beacons were still going.
Done that one too. Another classic stress reaction.
He was very patient & forgiving (as ever), and said I'd done the right thing on my touch & go. He explained to me the engine can gum up a little if it is idle for a while on finals but seemed happy I'd decided not to press on if I thought the engine sounded rough.
Nothing to forgive, since as he said, you made the right decision. I'd have said the same thing - and meant it - to any of my students.
Somehow I feel like I've let both him & myself down...
Ridiculous. You haven't let anyone down, and you instructor should be proud of your airmanship...and of himself for teaching you so well.
I get narked when I make mistakes but some of these were absolute howlers, so I left pretty annoyed with myself and I'm still feeling pretty ashamed with my last performance.
NOW you're being a prat! Stop over-reacting and feeling sorry for yourself. With the knowledge you had, you made the right decision. You also found out what happens when you get stressed. Learn from it, so that next time something scares you - and it will, since flying can be scary at times - you'll know to use a checklist for the simplest things, and double-check everything you do.
Now, don't start beating yourself up for over-reacting either.
I've done that too. There's something about flying that makes perfectly normal people go completely over the top about little things that they'd take in their stride in any other area of their life, especially in the early days. I don't understand why, but I've done it, most people here have done it, and yours is the umpteenth thread over the years started by student or low hours pilots who make Mount Everest out of a barely visible molehill.
Enjoy your next solo flight....and the next several hundred of them.