You learn by doing, not by being shown - especially by someone competent, who is not a teacher, is part of your family, is prone to frustration and close to a deadline!
Youtube videos can be good but you have to learn actively; pause and try it, go back if it didn't work and note down what was different about your computer / software / experience. Often there are small but meaningful differences in the way your screen is set up, that do make all the difference. Note those, set them aside and get on with the next thing. Later you can ask someone competent (at a relaxed time) or ask on an online forum and you'll probably either find it's because you're using version x and the demo used version y, or that you just need to know where to click to reach that function.
I do know how you feel. I feel this about tax returns. Every year I get it together to fill them in, face a question and think 'I have no idea! How did I deal with this last year? Why do I have no memory of that?'. So I started writing a commentary for myself, noting what information was required, or what decision I had made and why. That was very helpful. I do find the same with IT sometimes.
I'm in my 40s and had no IT lessons at school. We didn't even use email until I was at university. I used to do written work by hand, then type it up (when required) and couldn't imagine writing directly onto a screen. I've never been taught to use IT, I've just gradually got on with it, found my way round the things I had to use in the workplace, asked colleagues when unsure and so on. So my answer to 'how do you know stuff?' is that I learned gradually, by doing.
The main thing though, is to think about this in advance and practise at a quiet, stress-free time. Like most things, the experience you describe is covered under the motto 'to fail to prepare is to prepare to fail'.
Good luck!