whether it's someone you met under through dating, or a platonic friend who doesn't reciprocate your feelings beyond that, there's something about that person that drew you to them and attracted your attention and affection that resulted in you being infatuated regardless of whether or not they reciprocate
the important things is identifying that something and seeking it out in other relationships; what it that they made you feel? seen? secure? appreciated? was it physical beauty or creativity or sense of humor? no qualities are unique to any individual, a lot of qualities are actually social in nature and cultivated over the course of a relationship. it's easy to get over your feelings not being returned when you realize that there isn't a scarcity around the kinds of relationships you can have and the people you can have them with, and that your search continues rather than it having ended in "failure"
above all else, orienting towards gratitude that you got to have that experience and what you learned about yourself through the process instead of the negative feelings associated with rejection, because rejection largely stings because of perceived scarcity. the thing that's often unsaid but felt is "i don't want to find someone else; it would be so much easier if this person just liked me back" or some permutation. but as long as you live and breathe there are others out there for you to love and be loved by, and maybe some of them will be romantic partners but none of them have to be "the one who got away" unless you put them on that pedestal and only relate to them and yourself from that vantage point