Have something on the brain
Definition 1 of 1
Pro Tip 1/3Sustained Focus
Use when someone can't stop thinking about one topic over a long period. 
Lisa has had wedding plans on the brain for months now.
Pro Tip 2/3Emotional Weight
The phrase often suggests that the thought comes with worry or excitement. 
Ever since the promotion talk, he's had the job on the brain.
Pro Tip 3/3Informal Use
It's casual and more suitable for conversational contexts, not for formal writing. 
You really have soccer on the brain, don't you?