I've realised what people mean when they say they've lost interest in their job.

I used to chalk it up to meaning "I have lost my enthusiasm, and no longer do it with gusto, and am happy to just take the money and wait for retirement".

No. It does not mean that. It means "I can no longer motivate myself to do tasks in a timely manner. The tasks still need to get done, but now I have to put in extra time and effort into psyching myself up to do it".

You literally expend more mental energy, and it's stressful as hell when the deadlines come around each week