Hi, it's nice to meet you The best thing I can think of to say off the top of my head is... nothing happened to your intelligence. It's your self esteem and confidence that went away. You probably have so many self doubts that you make mistakes due to all that stress and fear of making mistakes. There is a sports term called a "forced mistake."
It's what happens when your opponent pressures you and creates too many doubts about your abilities and causes you to overestimate his/her abilities. Playing with your mind
Confident people make mistakes too but usually not many forced mistakes. Fear, self doubt, low self esteem... all these things cause you to make "stupid" mistakes.
I don't think I need to say who and what causes this for you
I'm sure MichaelD would have a great answer for you. Hope he reads your post.
Have you ever asked your DH why he married you if he's so smart? That is not meant as an insult to you. Hope you get this sorted out.