we just fell out of love after moving in with each other.
That's not correct. The honeymoon period ended, you didn't fall out of love because there was not love to begin with. Understand the difference between the "honeymoon period" or "new relationship energy" also called "puppy love". This is not love. Love takes years to grow and it grows slowly.
is that in around summer 2015, there was a girl at my local shop whom for some reason literally gave me the jitters every time I saw her,
This is likely human pheromones if you two were within a few feet of each other. Pheromones are real, and the theory is two people can have a "spark" if their immune systems complement each others, so their kids will be extra strong and immune to more things. However, pheromones sold in the back of Popular Science may not be real. I think pheromones work at a short distance, about 3-4 feet.
Compatible pheremones is at the instinctual level only, it has nothing to do with you two being compatible in other ways. Be VERY cautious here.
There is also what I call "energy compatibility" which works at a longer distance. While not provable by current science, plenty of people experience this across 50, 100 feet or more.
Ask her on a date, use the word "date" so there is no confusing. Tell you have this crush on her. Get to know her and see where it goes.
I have a long time interest in psychology, specializing in relationship dynamics for 30 years.
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