obsessive
Your brain is so messed up by one idea or image that you can't think about anything else. It's like your brain is stuck in a loop and you're the sad one who has to live with it.
He can't stop thinking about that video game. It's like it's haunting him.
She can't stop thinking about that song. It's like it's in her head forever.
He can't stop thinking about that girl. It's like he's been brainwashed.