Hi Wouter,
one thing first: you are nicely on the edge of the board, i.e. you are carving and not drifting.
second: never mind, the backside is a hard thing to learn. You have to get some trust in your board first. I also started doing push pull very deep in the beginning, which wasn't too bad actually. Here's one thing you should also try, because rotation is in my opinion the key. Try to get a slope which is not as steep as the one you're riding on the video.
Here are two videos, which might help. On the first and second you don't see any Laydown on the backside, but there's lots of rotation. If you noticed: even though there's no laydown, Petr, the rider is very deep to the ground. From there it's only tiny step to the BS-laydown.
The third video shows some good exercise: after you tried to do the Letdown, keep the body position, try to lie down without riding, just to save the position of your body.
Keep on trying and making movies. This is very important, because what you feel while riding is totally different to what you're actually doing.
If you have any further questions, write...
all the best from germany and keep up the good work.
Regards
Lambert
https://youtu.be/UX5bW3uyfrc
https://youtu.be/nR5t6xPDf2k
https://youtu.be/yfbHLZIqA30