I constantly feel stupid when I try to learn things

I'm trying to make a point of learning new things and I started with maths with I'm pretty okay at (I have a degree in physics after all) but I went back to like basic maths and Khan Academy's "by grade level" breakdown and 7th grade math and it was like "write this decimal as a simplified fraction" and I felt so stupid that I had to consciously think about how to do it. And in my head I started thinking like how I'm struggling to do something that 13 year olds are taught as someone with a physics degree.

I also feel the same way when I start to learn a new language and it's like "what, you don't know how to conjugate irregular verbs perfectly in this language you don't speak?" and it ends up killing most of my motivation for trying to learn things.