こにちは! I around the same level and I feel the same way these reviews can be rough sometimes… But we push through!
Your brain might be tired. I have days like that where I’m just getting things wrong left and right (like yesterday!)
The way I remember 心 is thanks to all the anime, games and manga I consume. こころ shows up in so many songs. So that stuck in my head instantly. しん was a bit trickier to stick until I remembered that it shows up in manga. It’s like the opposite of どき どき, when a characters heart “stops” in fear.
外 and 内 I have no problem remembering がい and ない but I still trip over そと and うち I’ve yet to come up with decent mnemonics for those.