I had that.
Are you eating properly? After my husband died, my friend kicked my butt, and made me eat properly - Weetabix and milk for breakfast, lunch, dinner, with veg, fruit and carbohydrates.
I started thinking more clearly.
Unfortunately, I'm also depressed and demotivated, so it makes me not want to eat, and it's a bit of a vicious circle. I'm saying this last bit just so you know that I understand how difficult it is in reality, even when part of the solution seems so straightforward.