I can fall asleep almost anywhere. I routinely fall asleep in the break room at work. Once, I was helping a friend fix his car, and I fell asleep on his garage floor when he went inside to get water.
But in a hot metal tube tearing through the sky, with my neck all kinked? Get out of here, man.
I started wearing ear plugs and an eye mask every day. Eventually I got so used to it to the point that 1) it feels comfortable and 2) my body associates those things with sleep.
When on a plane I just put the ear plugs and eye mask on, and my brain just knows it’s sleep time.
Also, not all neck pillows are created equal. I found the biggest factor is the pillow having straps to secure it to the head rest. It will do the work of holding your head and you won’t drift sideways as you fall asleep.