Oh god, I've had this - just once thankfully. I felt gripped by a madness I couldn't escape from. It must have gone on for about 2.5 years.
It was during a time of turmoil after leaving my husband and moving away. I bumped into this man regularly (as an acquaintance, nothing else) but felt nothing until one day I realised I was admiring his fingers. I thought to myself, "That's interesting, I'm obviously interested in this man, but I never realised it till now."
Then it was like a switch. I was obsessed. I ended up asking him out because when I initiated conversation he was eager to talk. In retrospect I think he was lonely, and just enjoyed chatting. But in my mind, he was "the one" and I was "the one" for him. I dreamed up all sorts of scenarios, I even started looking at my house and asking myself if he would be horrified at my garden or my front door. The good thing was I suppose, I did an awful lot of sprucing up for a while!!! After I asked him out he avoided me, and I went along with that but unfortunately I then started bumping in to him a lot in town when he was taking his mother shopping. And since she knew nothing of this I got dragged back in, and whenever I saw them I'd make a beeline towards them and talk to her at length just to be in the same space. I even went on to 192 and found out where he lived. I knew then that I'd drive past, but managed to hold off for about a year.
The whole thing was agony. I desperately wanted to stop but couldn't. I cried with the torment I felt.
In the end I met his mum once on her own and asked her for a coffee. She was rather dismissive towards me, and it was like a switch again. Off. I didn't pester them again, and when I did come across him I just said hello and kept on walking. I was no longer interested.
His mother died recently and I gave him a card, but that was it. Then he seemed on a couple of occasions to be trying to talk to me. But I knew that if I got back into talking it'd happen all over again, especially since his mum wasn't there any more. So I purposefully pretended I hadn't seen him.
Since his mum died I've found myself on occasion wandering into fantasy land, but it's nothing like it was and I know if I keep away I'll be safe.
I always thought it was just me, and that I was suffering from some kind of madness. I lost a friend because of it - she thought I was a nutter! And I was! Utterly mad.
It was a completely irrational obsession. I'm glad I'm rid of it and I feel so so much for anyone in thrall to liminence. It's a horrible thing.