Thanks for sending us the questions!
Always fun when people reach out for some guitar tips and tricks!
What you’re describing is pretty common, and most people have been experiencing similar problems when playing guitar.
There are a few things that you can have in mind.
Make sure you cut your nails fairly short so that the angle of the fingers (tip of the finger) presses down the string in the right way, just like in the video that you sent.
If your action (string height) is a bit too high, this problem of “missing” the string so that the finger gets under the string will happen much more often. So it’s really good to go to a music store and try a guitar with a really low and comfortable action, which will make it much easier - and you’ll not have the same problem as much. Most guitars can be adjusted so make sure you give it to a luthier or maintenance and repair person to help you lower the action if it’s too high.
Everyone has different opinions on how wide the guitar neck should be. I’d suggest you to try acoustic guitars with different measurements when it comes to how wide the neck is, cause it might be much easier for you to have a wider (or smaller ) neck.. try out different ones and see if this problem occurs or not.
Strings. Try different types of strings, to see if this helps. Coated strings are much more slippery, which can cause these problems that you have. Phosphor bronze strings that are not coated might be better for you! (I use phosphor bronze strings)
The video exercises that you sent are ok, and will help - but make sure your right hand are located over the sound hole (and not like the guy is teaching in the video).
Hope this helps! Kind regards/ Emil