I'm not obsessed with my ex, but if an opportunity arose to fuck her again, I'd take it. Just to find out if she is still a nasty slut, if nothing else.
We don't communicate, and only see each other at family events (birthdays, graduations, etc.), but she has been very friendly when we happen to run into each other, inviting me to dinner (not privately, other family was going also) or some other event that was upcoming.
So it's unlikely that hookup would ever happen, but it would be fun to find out if she still fucks and sucks as good as she did 40 years ago!
In any case, I still masturbate and relive a memory or fantasy of her on occasion.