Mental mathematics is kinda sweet. I use to do all those far back then in high school as well. But I don't see any reason to do them now 😂should I say because I don't deal much with calculations or I am just lazy or I don't see the need to? Whatever, that's by the way. Yea, lack of sleep could also be a factor. Why don't you give yourself a proper sleep. I know it not easy, but it something you have to. At least, gradually.