I said something that made my husband feel stupid
What did you do?
He was helping me study and I told him he could not test well.
You can have observations about your partner. But don't insult them for helping you, you're the one who needed help, not him.
It hurts me that he thought I'm calling him stupid.
It sounds like it was a misunderstanding. I think the best thing to do is explain what you meant and apologize.