Maybe I just need lots more repetition: I can remember what to do OK, but I don't have time to think through it. I should be doing the right thing quickly and without thinking, almost as fast as a conditioned reflex.
This just means you have to do 2 things.Possibly another thing is that it's not really possible to practice defences on my own as I can practice punches and kicks (to an extent) on my own.
1. Trust the technique
2. Build habit when means to do the movement over and over while visualizing the kick to the head.
What I would do in your shoes is to find someone who can do the kick to my head without landing the kick.
Tell them that you want to just see the kick a few times without blocking so don't land the kick. Don't kick the head but send the kick in the direction of my head. I just want to get a clearer picture of what is coming at me so I can better visualize for when I train on my own.
Your reaction time and confusion are probably because you don't have a good read on the kick and as a result you aren't reacting as you training. Here's a person experience:
I have Muay Thai sparring partner that I train with and one day he threw an overhand that kept getting in and I couldn't figure out why I was getting hit. So I asked myself "What am I not seeing?" I stopped the sparring session and asked my sparring partner to throw the punch but don't hit me. I just want to watch everything as the punch comes in without having use any brain power on thinking about reacting. He threw the punch 5 times and it helped me get a better understanding of what I was dealing with.
I often tell him when I see a strike and when I don't see it. If I see a strike come in and get hit because I didn't react, then that's fine. It's when I'm looking and I don't see the strike that is an issue. I can't react to something I can't see. You may be having issue. You see that a kick is coming, but you don't really see it as you should. Your body doesn't recognize the kicking movement and as a result your reaction is a confused one. In other words, your eyes see the incoming strike, but your brain doesn't understand it.
Your brain has to understand the incoming strike that your eyes see. For me it works to do what I explained above. It may help you. Just make sure that you get someone who has good control of their kicks and can do the kicks slow enough. You don't want them to do a fast kick because you are trying to see the kick so you can better understand the movement of that kick.