Hiya and thanks for adding more background.
Why does the feeling keep coming back?
I'm guessing 'cause it's a nice feeling, to think there's someone wonderful out there and that you already know them.
Truth is, there is someone wonderful out there, and there are potentially several wonderful or proto-wonderful someones out there, but you either don't know them yet or you haven't cottoned onto their proto- or full-fledged wonderfulness.
Because you're busy living in the past. And you're busy with grad school and with life of course.
Feelings just kinda happen, and they can be good, or they can be sucky (to use the technical term
) or somewhere in between. Deflect and delay, I say. So - deflect your feelings by continuing to be and remain busy. You need not fill every waking hour of every day, but it seems that your mind wanders at times, and it inevitably wanders toward this dude. So! Give it a few other places to wonder. I dated a guy, egad, it was in Law School and he was already a lawyer, and he had spent Law School, every single day, he would take an hour out (usually this was lunchtime) and would read a completely unrelated book. I think he read a bunch of classic works, too, so he went through Jane Austen and Thomas Hardy and the like. One hour, no more and no less. The clock would start at noon, read, done at one. He read a ton! And yes, he passed the Bar.
You can also deflect with physical activity. My mind tends to wander when I walk or run, so I do the music thing, but I've also found that if I am doing a lot of heavier physical stuff, I do need to concentrate on that. Plus you get lovely endorphins and a sense of accomplishment (which can also come from the reading trick).
Delay as in, time's gotta pass. Your studies get closer to an end, you get older, your brief time with this dude gets buried deeper in the past, etc.
Yanno, here's a question. Are you close to being done with school? If you are, it's possible that the upcoming transition is giving you anxiety, and you may be coping with that, subconsciously, by thinking of a pleasant past fantasy. Hmm.
Or if I'm wrong, I'll return my psychology degree to the U-Give-Internet-Advice University Degree
store and see if I can get my money back.