You may be nostalgic for a possibility that never became an experience.

You can miss something that never happened. The job not taken, the city not moved to, the relationship that stayed potential, and the longing is for a version of a life that only ever existed as an option. Because it was never lived, it has no ordinary days in it, no disappointments, nothing to make it real enough to be over.

If a possibility keeps returning, try imagining it in full, including the dull and difficult parts it never had to have. Often it deflates usefully. If it survives that, it may be telling you something about what is missing now rather than about what you should have done then.