It's called road hypnosis or highway hypnosis, commonly known as white line fever. And trust me, it's not just you.
It's a combination of retinal fatigue, mental fatigue, the sensations of constant movement, the hum of the tires mixed with the rhythm of the engine, and those broken white lane markers zipping at you for hours on end. White line fever has been around as long as the white lines have.
What you need to do is leave earlier, and take more breaks while driving. The leaving earlier will compensate for the more breaks.
I drove medium distances for a living for a few months, and I've been on more than a few dozen half-day and multi-day road trips, so I know what you're talking about. And unless you take those breaks every couple of hours, you will (not might, not could, will) succumb to the white line fever. You don't want to do that.
The breaks don't have to be long, but they do have to be every two hours. For every two hours on the road, you need at least (minimum) ten minutes of break time. Break time means get out of the driver seat and do something / anything else for ten minutes. Drink a coffee, smoke a cigarette, get a BJ, take a shower, write a letter, compose a sonnet, whatever. As long as it isn't driving, it's a break from driving.