I believe there is a two-word phrase for something that is always just out of reach for you and which you cannot ever seem to get. (It is not Tantalus or anything having to do with Tantalus, please).
It’s like a girl you like and want to date, and it almost happens a bunch of times but never materializes. You’d say, “She’s my __ __.”
I keep thinking dark horse, but that’s not it. Any help?