I find my brain gets tired really quickly when doing things that have multiple complicated steps or things I find tedious/not stimulating enough. If I’m locked on to something, I suddenly have enough energy to do it for 4-6 hours straight. I would suggest trying to do those things in different ways. For example, my friend uses Khan Academy videos to learn math and just basically (politely) ignores her prof. The videos are usually engaging enough for her. For writing, I always have to break it down into chunks and do it piece by piece with breaks in between. I honestly don’t think you have a lack of intelligence. I think your brain just works differently and maybe doesn’t like doing things that aren’t stimulating in the exact way it needs them to be. Which is legit if you think about it.