>>I don't disagree, but when I've tried this I just find myself doubly exhausted: once (mentally) from the work, and again (physically) from the gym.

When you walk into the gym you need to stop thinking about work and start focusing on your workout. For me, this comes in the form of planning which exercises I'll be doing, how much weight I lifted the previous time for how many reps, and how much I can increment it. And then during the lifts, I focus on the mind-muscle connection to make sure my form is correct and I'm activating the right muscles.

Trust me, when you do it like this, it's incredibly relaxing from a mental perspective. I actually look at my gym sessions as meditation time.