Is it unhealthy to imagine that you're still in a relationship with an ex, or even somebody whom you've never dated?
Have you ever done it?
Do you know people who have done it?
That falls dangerously close to obsession. I myself have never done that, all my ex girlfriends have always stayed just that- ex girlfriends.
I knew one girl who did that with me. She was a nutcracker already, but that just made it worse. Called me nonstop like we were dating still, kept texting me about marriage and kids...showed up at my work a few times before management threatened to have her arrested. Freaked out months later when she saw me walking in town with another girl. Screamed that I was a filthy cheater...
Freakiest year of my life. My wife refers to her as the Dublin Banshee.