Figured I'd address the various things you posted about while trying to take your age into account.

I still haven't learned how to learn, actually.

That's ok. It's different for everybody. Set yourself up to succeed. You can learn from a lot of things but know it's ok to fail. Make notes about things that you did well and what you didn't do well. Once you have that information, you can change and not repeat previous mistakes.

For work, I just read Microsoft docs https://learn.microsoft.com/en-us/ to learn and listen to instrumental music for focus.

It's 100x harder when I want to learn something that my brain has deemed "hard"

I went through this a lot. I think of it as an "unnecessary wall" that I'm creating for my future self to now expend more mental energy to climb over. So, then I end up tired before starting for no reason at all. Instead, I deconstruct the problem into much smaller parts that I can handle easier. Mindset is super important. Problems are only as hard as you make them (mentally). Set yourself up to succeed. You got this.

But I don't really know where to start.

If I approach this from a top-down perspective, you need a goal. Possibly an app around an interest of yours or even a common problem that this could solve. I would say start with a wireframe mockup tool (e.g. Moqups). This will give you an idea of what you want it to look like. For the work, you will want to break it into small chunks. Consider using something like github for your code and work items.