Preying or praying on one's mind?

[midtowner]

I was explaining to someone at work today that I hadn't been sleeping because I had so much on my mind.

I've always thought that things were "preying" on my mind, but I started to wonder...

Can anyone enlighten me?

You're right, it is "preying"

Preying on your mind is correct. The thought is hunting (not haunting) you, following you around like a predator stalking its prey.

Thank you! That's what I thought, but I started to doubt myself...