I just stare at my desk, but it looks like I'm working. I do that for probably another hour after lunch too, I'd say in a given week I probably only do about fifteen minutes of real, actual, work.
Edit: I was slow to catch the reference. I'm avoiding watching that movie again because, as much as I loved it the first time, I know my brain would just use it to reinforce the idea that my job is a joke.
But to fix the root cause, you need to have activities outside of work, that you do with friends you don't know from work, that you're excited to get started on when you're done for the day - music production and lifting are what I do. Working remotely is another way to fix it, but your company might know about this and ban it like mine did.
I doubt I could cope with what you describe.