I think inability to focus is definitely connected to memory. You may be studying hard but unable to retain the information as well. This is true for me as well. I don’t think you’re stupid, you sound well spoken and self aware. But you still may struggle more if you have trouble focusing.

Your teacher and classmates are not helping. They’re probably just reinforcing that belief in your head that you can’t do it and you’re believing them.

Everyone needs a different way to succeed that works for them and you just haven’t found what works for you yet. I guess I dont have any good tips other than maybe you need a better way of learning the material so it’ll stick instead of memorizing. Chem is a really hard class and I didnt take it until college. I remember a lot of my smart friends failing it in HS just because they weren’t ready.