Why can't you stop thinking during practice? Because you’re a human being! Our brains are programmed to constantly process information. If we could stop thinking at will, we wouldn’t need to learn to meditate.
Give yourself time. With practice, you’ll learn to access the switch in your head, turn off the noise, and be in the moment.
When you struggle, bring your attention back to your breath. The sense and duration of calm will begin to increase with practice.
Learning to deal with thoughts as they come and go is a key part of the attention training. We’re learning to deal with the human condition. It happens to everyone.