Before going to sleep, try counting backwards from 100 to 1 by projecting an imaginary white numeral onto a blackboard in your mind and see how far you get before an interrupting thought invades the process.
Eventually, you'll be able to do it easily.
Next, recite mentally all the multiplication tables.
Go over each product taking one number at a time and recite its product.
One times one is one through to one times nine.
Then two times one through two times nine, etc.
See if you can go through the entire multiplication table without having your mind wander off.
The progress you make can be easily marked by where you strayed off in the multiplication table.