I had it in my teens after my first "real" relationship ended after 2 years. I stalked him and his new gf for months, I was lucky not to be arrested, looking back.
The fact that he kept sleeping with me "for old times sake" didn't help, obviously 
I told him a load of lies to try to get his attention and love, including making out that he'd got me pregnant and I'd had a miscarriage, amongst other horrible things. I am truly ashamed of what I became during that time. It was like a psychotic episode. I felt like anything I did or said, if I got him back, it was justified.
Then I had a bereavement and it just seemed to be like a bucket of cold water over me. I was able to move on.
Unfortunately the relationship models I learned as a child were very unhealthy and I had very little sense of boundaries. I was also spoonfed romantic tearjerkers and the idea that "if you love someone then it's meant to be and you have to pursue them to the point of death", people dying of broken hearts, and especially the notion that "if you just love him enough, he'll love you back eventually."
I have had plenty of sexual crushes since then, and I've been fannystruck over a new bloke a time or two, but I've never felt anything remotely like that obsession again, thank god.